Wheezing is not in-and-of-itself a dead-on indicator of a respiratory infection. The classic symptoms of a respiratory infection also include bubbling mucous around the mouth and/or nostrils, and gaping of the mouth. Before so many other good books on Ball Pythons were published, the old "Bible" for Ball Python enthusiasts was Philippe de Vosjoli's "Ball Python Manual." If you have a copy, or access to one, the book contains advice on how to accurately diagnose, and treat such an infection - as well as the recommendation to see a vet.
If this is a respiratory infection, you should be able to expel some of that bubbly mucous from the mouth and/or nostrils by gently grasping the snake by the head, placing your thumb underneath her chin and throat, and gently rolling pressure towards her nose with your thumb. If you don't see that bubbly mucous, or any gaping of her mouth while she is left on her own in her cage, she probably does not have a respiratory infection.
I know we all get spooked by a wheeze. I let my oldest female get too dry one winter, 2 years ago, and this caused her to whistle. It wasn't a wheeze exactly, but the sound made me worry. I talked with the vet quite a bit about it, and what he told me was just that she had gotten too dry. Humidity is a strange beast sometimes; too much can be just as harmful as too little. The vet related this to our own human problems with humidity in the winter - if we get too dried out, all of the mucous and junk in our noses can get dried out and hard, and that will cause us to whistle a little too, except that we usually try to do something to change that as soon as it develops. Our snakes can't just fetch a tissue and blow their noses.
If you don't already have a favorite spray bottle for misting, make sure you don't use one that ever had any cleaning solution in it, or fertilizer for that matter. Walmart sells some nice little spray bottles in the hygiene section, with the girlie hair stuff - but the bottles are really cheap, and perfect for this application, because on most you can adjust the fineness of the spray. They are also being made more ergonomic so your hand doesn't get pinched.
As far as your described setup (20 gal tank with newspaper and a UTH), I don't see a problem except for the lack of humidity. If you can mist daily or even twice daily, this would probably be alleviated. I don't ever advise keeping snakes on any kind of commercial reptile bark or wood shavings, as I have seen far too many issues arise with skin problems from these materials. I would say she would be better back on newspaper.
Another trick I like for managing humidity in the cage is that I use terra cotta flower pots for hiding places for my snakes. The same vet I mentioned before told me about this and I am quite happy with the results. The (un-glazed) terra cotta clay is good at absorbing moisture, and slowly releasing it over time. If you mist daily or twice daily, and make a point of spraying the terra cotta hide, the water you sprayed in the cage will evaporate a little more slowly because of what was absorbed into the terra cotta flower pot, before it was eventually released as well. Granted, to build these hides, I didn't just go to the gardening store and place the pot in the cage; I use a woodworking rasp or a ceramic-specific "hole saw" that fits into an electric drill to enlarge the hole at the base of the flower pot, and also to cut a "door" in the rim of the pot. The flower pot then sits upside-down in the cage and the snake is pretty happy. 
I really like "azalea" flower pots, because they tend to be wider and squatter in proportions - again something snakes really appreciate.
I would have to guess that between your female getting too dry, and as you suggested, too cool, this is why she is off her feed. Also, I am assuming that she got too cool and dry because you were trying to cycle her to breed. I don't know how cool you are saying she got, but Ball Pythons don't need a big change. If you think about the part of the world they are native to, which is not far from the equator, they wouldn't see a large seasonal change in temperatue, or even in daylight cycle length, though I have read many accounts suggesting a 10:14 day to night ratio to instigate breeding. What you do read most about however, is that Ball Python breeding cycles are heavily influenced by the rainy season. I would think that allowing your female to get too dry would negatively influence that cycle more than anything. My understanding is that Ball Pythons tend to get triggered to breed more often from a change in humidity than anything else. Unfortunatley, I have not found this out for myself first hand, but I have done a lot of reading, both here, and in published works. I can tell you that with the difference in light cycles, and perhaps a 5 degree difference in temperatures, my male and my female who are still housed separately get very "antsy" and excited now when I mist them. Any other time of the year, they would just sit.
Anyhow, there are lots of things here for your to consider. Also, read up on why so many people suggest 1500g as the threshold for a safe breeding weight for a female. Not everyone will agree with this, but you should read about the why's of each side before you make a decision concerning your animal (your investment). I know this is very long, but I hope this helps!
~Rebecca
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0.1 Dumeril's Boa '04 (Courtney)
1.2 Ball Pythons
[1.0 '05 Orange Hypo (Specter)]
[0.1 '05 Het Orange Hypo (Sylvia)]
[0.1 '03 Normal (Sue)]
0.2 American Pit Bull Terriers (40lb darling lap dogs:Brandy&Mara)
