Bad Battery? OR Over charged battery?

Okay so I’ve had my iPhone for almost a year coming up in June. So Very recently my phone has been dying really quickly, by quickly I mean I have to bring a charger with me whenever I go places.

For example, one day I was on my way to school and when I got there I was on my phone, within 10 min my phone died on 100%. So I held down the power button and it turned on but when it turned on it was on 10%. It does this all the time. So I just need help from my fellow techys on how to fix this.

Thank You!