Recently I had a similar problem, the battery was apparently discharging quickly and my initial thought was that the battery required replacing (18 months old).
At the bottom of this page:
http://www.apple.com/batteries/iphone.html
it mentions going though an entire charge cycle once per month (charging the battery to 100% and then completely running it down). This is something I rarely do, I usually just top-up the battery.
Letting the phone run down until it switches itself off and then fully recharging to 100% using the wall charger worked for me.
My explanation is that the battery meter lost track of the charge in the battery. Allowing the battery to discharge down sufficiently so that the low voltage circuit shuts the phone down, recalibrates the meter.