iPhone Heats Up And Battery Then Drains Quickly

My iPhone 3GS has recently been heating and it is in an otter box case but this is not the case because I took it out of the case and it still feels overheated. The battery also doesn't last, it only lasts for about 2 hours then it will just die. The battery has been replaced but it doesn't seem to have helps it. I was wondering if the battery drains because of the overheating or if it's something different. I have restarted it by holding the home and top power button but it hasn't helped it.

Please help!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!