never play through the pain, if you feel pain and you need to stop cause it hurst too much, you've already gone too far.
You're probably working your way to a nice "tendinites" I'm not sure how it's called in english but your screwing up your tendons :)
I've seen players out for 8 months because of it.
Feeling warmth is ok, feeling pain is just not right.
If you do long periods of practicing, just do shorter periods of different exercises. If 30 minutes hurst, do 15-20 minutes. Alternate between exercises that requires streching and let's say chords progression exercises or pentatonic scales exercises.
It's only a matter of changing the stress in your hand. Also, just like school take breaks. Don't go past 45 minutes without a small 10-15 minute break.
Personnaly I never did practice sessions that are more than 1 hour. I always try to mix practice and improv in a 1/1 factor. If I practice 30 minutes I jam over various backing tracks for 30 minutes. Practice is good, being able to jam over any backing track is priceless :)
Just stay away for pains, warmup first. Nobody is the same way so one may be able to play 3 hours straight but another only 30 minutes. I'm more of a 30-45 minutes kinda-guy, pass that I don't have the concentration and I slowly mess up my execution.
Hope this helps.
Btw: I play with a short strap, gives me less pain because my wrist is not bent as much as keeping a low strap.