just updated to 6.1.3 and my battery now don't understand iPhone 5

I haven't had any battery issues at all with my Iphone 5 until I updated to the latest software 6.1.3. Usually my phone last all day and I didn't even use it and the battery life was at 40% so I turned it off. When I got off work I turned it back on and it was in the red at 20%. Why is this happening now when there was no problems before? The whole reason I upgraded from the 4s to the 5 was for battery life, but since the software update battery life is sucking!!