Battery Suddenly Draining very quickly?

I'm on the 5s, Latest software and yesterday and normally my battery lasts me all day, plug it in at around 10pm (with about 30% battery) and I've never had an issue, I'm on wifi all day at work and use it, i would say, less than the average. BUT! Yesterday, I noticed it was at 20% at 5pm and today it was at 20% at 1pm?! The usage stats claim the same use I normally see, but this time my batteries completely drained?! No new aps that I know of, Need help!!