Phone hot after 10 minute conversation

I looked around and haven't seen this problem anywhere here yet. I've had my phone since November with no problems except this one. Usually I have short phone calls, but anytime I end up on a conversation of 10 minutes or more, the phone gets hot. It actually gets to the point where it's uncomfortable to hold the phone. At least the incoming sound is loud enough that I can hear as I hold the phone far enough away from my ear. This only happens with phone conversations. I can text for an hour with different friends, use the browser, google maps -- you get the idea. Does anyone have any suggestions?

That's what I don't understand. I can have google maps on for a half hour, and it's fine. Guess a trip to AT&T is in order....