According to Apple discussion that iOS 4.3.1 is causing some iPhones to overheat and short battery lifetime.
Many users are saying that battery life on iPhone has decreased dramatically, and it will occasionally overheat while in use.
Here's what they said at Apple discussion:
My iPhone 4 for the last few days has started to get very hot in use and the battery is gone very bad. Leaving the phone on standby can see the battery drain nearly 20% in an hour. This is gone to the stage where I have to turn it off at times just to save battery during the day. It only began to happen when I updated to iOS 4.3. I updated to iOS 4.3.1 yesterday and it still has the same issues. Anyone else having the same problem?Some users adviced to disable Exchange mail accounts to turn off notifications of apps and etc. Actually the solution is quite simple than all these advices.
There is a big chance that some corruption may be occurred while restoring your iPhone from a backup, which makes your iPhone consuming the battery and overheated.
Simply you can fix this issue by restoring your iPhone as new instead of from backup.
Sure this will be much longer to re-sync contacts, apps and etc, but it will fix your overheating and battery life issues.