Battery discharging while plugged in

Myntekt

Honorable
Aug 7, 2013
43
0
10,580
Hello... I've had a LG Optimus G since last summer and it has always worked great. I've used the phone when charging and it had never given me any problems.
But today, I've plugged the phone in and started using it, first playing some games and then watching a movie, and the battery discharged from 30% to 10% and now it is staying at 10% (goes down to 9, up to 11%).
Normally, if I used the phone while changing it would charge, at a lower rate but it would.

Can someone help me out?
I'm running Android 4.1.2 with no mods or whatsoever. The phone has been rooted once but I unrooted it.

Thank you.