iPhone 5c battery won't hold charge

In the last 2 weeks my phone charge in not even lasting 3 hours after being fully charged. I am not even using it but when I do it will drop between 10-20% in about 10 minutes. I have gone through all of my settings to make sure the ones that have previously been the problem for causing this, but none of them are on. Have let phone go completely flat and then recharged but it is still not lasting long. Any advice is appreciated