If you also have a Mac, have a look at the Coconut Battery software at
coconutBattery 3.6.6 - by coconut-flavour.com
I've used it to show the health of my iPhone's battery. You'll have to measure it a few times over a few days to get a good average value as just one measurement can be misleading. If the battery health has dropped below say 80%, that may be an indication of a bit of old age creeping in on the battery front. Mine's at 60-70% depending on when I measure, and that's after more than 2 years of use on an iPhone 6. Once the store's supply increases, I'm getting a new battery put in at the Apple store's reduced rate (€29 over here).
Could you let us know what model iPhone you're using, and how old it is? Did you get a new one, or a refurbished one? If it's only few months old, I wouldn't expect it to have decreased battery performance already. In that case, G0ULI's advice will be worth pursueing. Have a chat at the Apple store or give them a call.