Squirming at my breast

My baby girl and I have just worked out a comfortable latch after three weeks of pain. But now a new problem arises. After about 5 minutes of nursing she starts jerking her head back and squirming and kicking . I don't know if it's cause my milk is flowing too fast or if it's cause it's not flowing at all. I did notice however that previously she used to feed for about 20 minutes but since we passed the 4 week mark she's only been feeding for about 10 minutes . Her poop and pee output hasn't changed though. Any clue what's happening? Or how I could possibly find out what's wrong?