Hi,
Either your cpu/gpu is being heavily used or there is a fault in your battery (or you are simply charging it). I assume that your battery life is not great, correct?
Install CPU Spy Plus - Free and see how often your CPU is in deep sleep (should be well over 30% of the time). If this percentage is low then something is keeping your phone's processor busy. If it is high, and the battery is not being charged, then I would try a different battery (perhaps you can borrow one for a while from a local shop).
Regards,
Eric.