What is wrong with my phone?

In the past couple weeks, my phone battery has been acting weird, restarting itself at complete random times, turning off at 40%, etc. The screen on the left side was also popping up, a pretty large amount for the screen and so I figured it was a swollen battery. I have an IT person in my family who fixes computers for a living so i bought a $30 repair kit off amazon and had him replace it. Immediately after the phone still didn’t work right, it started restarting itself and the battery level would change almost everytime it did. I left it in to charge overnight and it only went up 3%, and when I unplugged it to go downstairs and plug it in, I tried to use it and ever since then it consistently restarts after maybe 2 minutes of use. Been plugged in ever since and hasn’t gone up any at all. Also just inspected the same left side of the phone that I had problems with before and it has gone up already even thought itwas completely level again after changing the battery.