iPhone 8 battery issues.

So I charged my iPhone 8 up to about 88% and didn’t use it and looked at it 10 minutes later and it had dropped to 32%.... I thought it was odd so I plugged it back in and left it for 5 minutes came back and it had jumped to 71%. I did a hard reset and it then jumped up to 73% and as I write this ( which took about two minutes) it’s dropped from 73% to 69%, is my iPhone faulty??? I’ve only had it three weeks!!!!