I think something is wrong with my snake's belly

[Agmcmurphy]

I have had Maddox, My ball python since Oct. of 2012. She is 4 years old, and 4 1/2 feet long weights 4.2 pounds. She eats 1 medium feeder rat (live) every 2 weeks. Her last meal was 5 days ago. I have tried to get her to eat frozen, but seeing as she was always live fed, she refuses pre killed prey. Also as a side note I have tried feeding her a little more often, but she will not eat more often than the 2 week mark. I have her in a 40 gallon breeder tank with screen lid. I fasten it to the top with 4 black metal lid clamps. For heat, on the hot side, I have a dome light, (white incandescent 75 watts, and up until recently I had a Zoo-med heat mat underneath. I cover the bottom with Repti-bark. The substrate is about 2 inches deep. I measure temp with a heat probe that stays in the tank. I also have stick on thermostats on each side. The room temp is 75 degrees. She has a half log hut on the warm side of the tank. I dont have a hydrometer, so i am unsure of the exact humidity. I do how ever spray down the tank and my snake gets semi regular baths. She has an ex large dog dish for water. She is the only animal in the tank. I normally hold her at least once a day for at least a few minutes. I do not handle her for 2 days after feedings. The only medical info I have on her is a clean bill of health, also we probed her and found that she is a she.

Today I noticed she had some redness on her belly. Normally i will see some redness right around shedding time, but she has shed recently enough so that should not be the case. After her shed the belly area cleared back up like usual. I checked the heat in and on the substrate, and the tank itself they were fine. how ever now i see that it appears she is burnt. or possibly her underside is being irritated by something. I have not changed anything in the tank to cause an irritation. I did pull the plug on the heat mat today, just in case. I am looking for some pointers on why she would show up red underneath. Also, i will post pics.