Battery drained - changed battery and front screen

Hi everyone! I have an iPhone 6S that I did some work on:

- due to Apple's faulty batteries issue, I changed the battery a few months ago with no issues whatsoever

- three weeks ago my phone fell and the screen cracked so I got the pre-assembled screen and changed it myself. It all worked well for a week or so and then my phone started dying extremely fast going from fully charged to 10% battery life in 30 minutes or less. In addition it goes in to reboot for no reason and then shows battery levels going down as well.

I re-opened it to check whether I left any connectors loose and this was not the case, and none of the connectors seem to be damaged.

Please help!

Thank you,

Isabel