baby stops during feeding and shakes her head around?

thankthesox

i've been starting out with breastfeeding (baby is in NICU still) and it seems like she will stop at some point and get fussy, and will shake her head around like she doesn't want the breast but then whe i put it in she latches on and sucks a lot but then gets upset again.  i was using a nipple shield and there was definitely still milk in it.  we wound up giving her a partial tube feeding because the nurse didn't think she'd gotten enough to eat.  but now i'm wondering if maybe she had emptied that breast and was just trying to tell me it was time to switch?  i've had a lot of trouble with pumping and it's hard for me to tell when it's been emptied - i usually just watch until it stops squirting out but with her i can't see what's going on to know that.   anyone with more experience BFing know if this is what they do when it's time to switch or would she just keep going anyway if she was still hungry?