Cat,
It just happens that I had asked my OMS and Neuro about this last week as this has been going on with me for awhile. They explained it to me as 'overworked muscles'. It's like when your exercising and you over do it, the muscles actually get hot<warm> and hurt. With tmj, it usually means clenching, grinding or stress. I'm not usually a clencher and I never grind, but, I did start noticing that when driving rush hour, I tend to clench.
If it persist, I would still have it checked out.
Hope this helps.
Paulette