8 1/2 month old refusing feeding after surgery?

My daughter has been exclusively fed until she was 6 months old. Now she has one meal of stage 2 baby food and the rest of her meals are milk (either in a bottle or from me). She had to have surgery on her spine/neck and her nursing sessions have gotten shorter. The day after the surgery, she would eat for 15 minutes total. The next two days she would have 10 minute sessions. Today is the 4th day post surgery and her last nursing session was 3 minutes on one and 2 minutes on the other. I still have plenty of milk, but she stops eating after only a few minutes. This weekend I can pump and then feed her if I need to, but on Monday, I will be working from home and won't have the time to pump and then feed her. I have frozen, but it takes me so long to thaw it out that she is furious by the time it is ready. I would have had plenty of refrigerated milk, but the hospital refused to let me store it in a non-frozen state. Any ideas? I know she's hurting, but she's actually more mobile now than she was four days ago. She's rolling over again, sitting up, wanting to bounce, turning her head, and she'll sleep on her side. I can't figure out why her sessions are getting shorter and shorter when her pain and stiffness is getting less and less. I've tried other nursing positions and nothing seems to work. I'm thinking of just giving up on nursing and doing formula instead. Please help.