baby gets fidgety while feeding

Usually in the morning, LO feeds with no issue because I'm engorged but usually around the evening time, she starts getting fidgety while I'm trying to feed her. I don't know why she does it, at first I thought it was her way to expressing that she was full but she would start doing it five minutes after being on the boob, and she can feed for about 15-20 minutes at times, so I know for a fact that she's not getting full. I'm trying to figure out why she gets so fidgety? Does anyone else have this issue?