iPhone Battery Issue

I use my iPhone as an alarm. Normally, I have it at around 60-80% of battery left right when I go to bed. When I wake up... it's usually a few percent below that. Lately, my iPhone's battery has been acting strange. The battery drops really fast. Yesterday, the battery went down from 70% to 7. Checking email at school for like 2 minutes and listening to music is all I do with my phone, but it still drops like crazy. Have any idea what's going on? Thanks!