My dd is 7 weeks old and is bottle fed formula. Recently, ive been struggling to feed her as she starts crying and thrashing her arms and legs around during feeds. She does this about 50% of the time and the other times she feeds normally. It's very frustrating as she will cry/scream and thrash around and then suck enthusiastically for a few seconds then cry again. We have Avent bottles and she's on the stage 2 teats so I don't think it's an issue with the milk not flowing fast enough. I can't work out what is wrong with her 
Any ideas?