IPhone 4 plugged in but not charging, shows plug symbol instead of lightning bolt?

I just dropped my iPhone 4 on the ground while it was in a bumper case not 30 minutes ago. After I picked it up, it was rebooting itself on its own. After it turned back on I noticed my battery had dropped from maybe around the 50-60% range (I don't exactly remember) to 7%. I freaked out and ran to plug it in. As I plug it in, it made the charging chirp like normal but did not show the lightning bolt symbol, rather the plug symbol as if it were fully charged. I quickly shut it off, took the back off, and took the battery out and reconnected it; I turned it on and the battery showed 9% after that (wtf?). I had it plugged into the wall where it had the plug icon and now have it plugged into my macbook where it still shows the same symbol. I'd say about 3-4 months ago, my phone did this same thing with not charging but without having dropped it; I had it repaired and they told me it was an issue with the connector which I paid to have fixed and it was fine after that. I'm not sure what to do now, please help! Its an AT&T 32gb iPhone 4. The version software says 6.1.3 (10B329) which I'm assuming is the latest, I always update to whatever is newest. What do I need to do? Will my battery drain while it is plugged in with the plug symbol? Please help!

Added (1). Also, it is not under warranty. I bought it off of someone after I ruined a blackberry and needed a new phone in the middle of my contract so an apple store would do me no help unless I wanted to pay $400.

Take it your nearest Apple store.