Why is my iPhone 6 charging backwards?

I've had this thing for almost two years. Its on the current IOS update. Its also "locked" because my parents couldnt afford the bill after 3 months and now i just use it on Wi-Fi. This week, i noticed it wascharging backwards. I looked up a fix and did a hard reset, as per recommended. Still, it has happened once every day this week and i'm getting concerned. I have to reset the phone each time this happens and it goes back to charging normally but i'm still concerned. Its probably the phone reaching the end of its reign, right? Just give me a clear answer, please.

Go to an Apple store and get the battery replaced. Should be $29 if it not part of the iPhone 6 free battery replacement lot. They will check your phone's serial number, and it out falls within the affected serial numbers, it will be a free battery replacement. You may want to call ahead, to see if the Apple stores near you, have the replacement batteries in stock.

Go to The mobile Service center