IPhone :: It Isn't Keeping Full Charge?

My iPhone 4 doesn't seem to be keeping a full charge. My phone never used to do this, and I'm not sure what's the cause of it. Every couple of nights or so, I'll plug my phone in to charge before I go to bed, generally the battery's down to 30% most of the time due to using it throughout the day for work and whatnot. Anyway, before I head to bed, I'll plug it in and turn it off. And when I'd wake up the next day, I'd unplug my phone and turn it back on, the battery would be at 100%, and it'd be good to go for the entire day.

But here lately, it hasn't been doing that. I'll plug my phone in before bed, turn it off, and when I wake up the next day to take it off the charger, it'll be at anywhere between 93-97%. And I don't know if it's just me, but sometimes it feels as though the battery's life goes down much faster when it's not at 100%, but maybe that's just my imagination or something. And when I keep my phone on the charger once I've turned it on, it'll get up to 100% after a while.

Like I mentioned before, this has only been a recent thing, and has started doing this maybe about a month or so ago. Needless to say, it's rather frustrating. It's not difficult to deal with, just a bother more than anything else.

Info:
iPhone 4, iOS 5.1