There is a condition called Burning Mouth Syndrome sometimes due to Vitamin B deficiencies but othertimes no known cause. Here is a link and you will find a lot more info if you google it http://www.bbc.co.uk/dna/h2g2/A2054602
I hope you can get some relief- it must be very unpleasant
If it is any consolation to you, I also have that feeling in my mouth. My tongue especially burns and there are many times that my breath is just hot. Right now I have a couple of sores on my gums, trying to decide if I should go to the dentist or continue to use the home remedies for canker sores. I know with immune systems low it is never good to let an infection stay around. Best of wishes to you. HOpe you feel better...it's not fun!