Overheating is usually a sign either of a faulty battery or a process that's running out of control. If you take the battery out and put it on a flat surface, can you rock it? If so, the battery's dead and I'd contact Samsung direct as they may send you a replacement without your having to send the phone away.
It would be unlucky to have two faulty batteries, but not impossible. You haven't been allowing the battery to go flat, have you? This would shorten its life considerably.
Overheating is never good, so I'd try to use the phone as little as possible until this is sorted.
