Why iphone is hot under camera after battery replacement?

I changed my iphone 4 battery last week. The battery is not original, My Iphone used to become hot in the area under camera a little while charging or using it alot, Now it is like before but the heat is more, Do you know which part is always heating the iphone specially when I charge it? (I know that battery itself never heats up, the only place is under camera)

That part should A4 CPU, and that' s normal nothing worried about
iphone.pandaapp.com

If you feel that it is abnormally hot, consider that the battery may be the issue. I usually try to buy my parts from reliable sources with good reviews. It could very well be that the battery is pushing too much current. You may try another battery that is of better quality. If that is not the case, check all your flex cables and make sure they're connect properly. Don't just check your camera flex, check all of them to be sure. Whenever I come across any issue with iPhones that I can't directly diagnose, to be safe I tear the whole thing down and rebuild it in its entirety. I ensure all connections are in the right place and stable, and I ensure all parts sit in comfortably (as it is intended to) and not forced in any way.