Do your Iphone suddenly empty your battery?
This just happend to me. A few days ago i updated to 6.1.2 , nothing wrong with that, pls do BUT something went wrong with my phone and suddenly battery-life was just 2 hours. My wife also updated without problems.
Connect to Itunes.
Make a backup (sync)
Reset the phone back to factory settings and after that sync back the backup you just did. Will tak an hour or so but should do the trick!