I had ruined a new pair of boots on the shifter side of the bike. After running in the mountains all week, I had apparently snapped off the heel guard, unnoticed. That-is until half way through Kansas in 105 temps. I smelled burning rubber at a rest stop. I THOUGHT my heel felt warm. Just thought is was because of the extreme heat. New heel guard, all is well again (except for a melted boot).
As for you just noticing more heat, I'm not sure. A more lean condition would produce more heat for sure. Could be injectors clogging. Could be winter fuels. And yes, I believe a faulty O2 sensor or cat could cause excessive heat.
KTM 400 EXC