I lose battery even when I don't use it?

I have an iPhone 5s (I don't have the iOS 8 update) and I had 100% and I turned off my phone then I turned it back on and I had 95% and now my battery goes by really fast even when I'm not using it. I have no apps running or anything and it barely started doing this! Does anyone know why and can you tell me how to fix it?

First, the phone is never really "off"; its just in standby. If it were off, it would never see you press the button to turn it on.

Second, batteries have an internal resistance so they will discharge a bit all by themselves.

Third, if your battery is getting old, it may not hold a charge as well as a new battery.

All of these things are pretty much out of your control if you are putting the phone into standby. If it gets worse, you might consider having the battery replaced.

Its not to say you don't use it the battery would stay up for good until you do again. It still does things while in standby, like if you are on a Wi-Fi/data connection it is checking apps in the background, like Facebook, mail, etc