i think it would be ok, if you just did it for a bit, like 5 minutes or less. they use to say no hot baths (had to be luke warm) but now the say just as long as you don't overheat yourself. my dr recommeded a heating pad for my sore back
You can check with doctor just to be sure, but I wouldn't if I were you. I had a similar question regarding whether I could use a heating pad on my back and I found this info from Babycenter.com:
http://www.babycenter.com/400_is-it-safe-to-use-a-heating-pad-for-sore-muscles-during-preg_510138_1001.bc
In short, it basically says you really shouldn't put the heating pad directly on your stomach/abdomen. I bet your doctor would let you know (I'm only 11 weeks pregnant and it seems like every day I think of an "is it safe?" question to ask my doctor).
Good luck.