Check your iOS is up to date
If that fails set up your phone again from a fresh start rather than using a backup
Alternatively you could contact Apple support and ask them to run diagnostics. If it is a faulty battery they'll replace it for you at a cost if you're out of warranty.
Be aware that one of the iOS beta updates did something similar to my phone although it was 32% was the number for me! It's ok now there's been another updated beta issued so it may be your iOS version