The fact that you use heating elements, such as the heat pad and the spot light don't really tell me anything, get a hot end and a cool end temp. I suspect that in a 20 gallon glass tank with a hot light and a heatpad that the temps in the cage are too hot unless you are keeping her in a cold room, so get exact temp readings.
Are you sure the snake is a female? If it is a male it my be thinking of breeding instead of feeding. Also with the hot light check on your humidity, my bet is that it is do to the 20% level instead of the 70% the snake needs as a minumum. I really have no idea how big your snake is but even a Hogg Island Boa will need more space then a 20 Gallon long tank. If the snake is more then three feet long it is time to get it into something bigger.
How long have you had the snake? was if feeding fine when you got it then stopped, probably a husbandry issue, the snake is too stressed to eat. If the snake has never fed for you it still may be husbandry but a medical issue could still be the problem.
The statment that you do not have a good herp vet is silly. Allentown is a good size area and there are plenty of vets to choose from. An avian/Bird vet would be a good choice, you just have to train them.
The first time I used the vet I have she did a fecal sample and said "Wow there is a lot of hair in this sample, what are you feeding this snakes?" I laughed my butt of and then started to educate her on Herps and she started to educate me on Vet Medicine and we found common ground in the middle. That was 8 years ago and she is now a great herp vet. But even a regular vet can due a fecal smear to see if there are parasites or other problems, that is not anything herp specific. If there are little crawly critters in the smear then that is bad, if not that is good. That way you can find out if it is a medical problem. Please visit my website for all kinds of husbandry and feeding answers. It is on my FAQ page and if that does not answer your questions feel free to contact me.
Thanks
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Jim Hopkins "Hoppy"
Hopkins Holesale Herps
Hopfam1@aol.com