My iPhone 6 keeps restarting and doesn't charge?

So my my iPhones battery wasn't working properly, it would die incredibly fast or sometimes even a couple minutes out of the charger! So I took it to get its battery replaced (not by apple) and my phone was working fine. It started getting slow, and I was on houseparty (an app where you can FaceTime multiple people at a time) and I told them I ll brb my phone is going slow. So I hard reset it and it starts turning off and Turing on like instantly. So it will be like working fine. Then just turns off. Wait a second and it turns back on. I never had this problem before either. I also noticed it hasn't charged past 3 percent and I've had it in the charger all night and it didn't go up a percent. The problem is still occurring. It has went up one percent so far so it's at 2.

Same

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