Hi D.,
Does your baby act this way when you put her to your left breast first in the feeding?
It could be that the left side has a stronger let-down reflex. If the left breast is typically the breast that you finish on, it could be your baby is simply full but is frustrated because she wants to suck more, non-nutritive sucking. If this is the case you could temporarily give her a pacifier until your supply is more in balance with her needs. If you need help with this write in again and La Leche Leage has good advice on ways to do this too. It could be that she is ready to try one sided feedings per feed. It really depends on what your supply is like, are your breasts have a full feeling much of the time? Or, do they feel empty or just soft?
It could also be that she may be frustrated that there is less milk in the left than in the right and wants more. There are ways of producing more milk in the left breast by nursing more frequently on this side to signal the brain to produce more in it.
I am sorry if this sounds complicated and confusing, it really isn't, but understanding the possible reasons initially, can be. Take a deep breath, your smart (I can see that as you have reached out for help instead of instantly throwing in the towel on BFing), you'll work through it. Consult a IBCLC Lactation Consultant, even if you have to pay cash, it'll give you peace of mind. ivillage.com used to have a wonderful LC online that would answer questions.
Gooooo problem solver! It's great your listening to your baby.
ps I agree a trip to a good chiropractor or cranio sacral therapist could help if there is something in babys body that hurts while being on the left breast. Get someone who is known for working with infants. They're out there. A call to La Leche Leage could probably tell you one other BFing moms like in your area.