When you go to the vet make sure the vet does a fecal exam. The monitor is probably wild caught and therefore probably has an overabundance of parasites. You should also increase the basking temperature to 130 degrees Fahrenheit or more, and the spot is big enough for the entire monitor to use. Make sure the cool side is around 80 degrees Fahrenheit. Some people recommend even lower.
I had a blackthroat that would not eat much and would throw up. I took it to the vet and after giving it medicine every 2 days the monitor got better. The difference from before and after was drastic. That was my first monitor that was not a dwarf type, and I learned a lot keeping it. I ended up letting a keeper in CA have it. I never heard if he ever got it to breed his female or not.
Oh Yeah, the hair on a mouse will not hurt the monitor. You may want to try to give it mice of an appropriate size.
This Argus grew from 18 inches to 36 inches in 6 months by feeding it 1 mouse everyday while it was small.

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Ed
These heat lamps make my electric bill to high.