I've been successfully BFing (using a shield, LO apparently hates my nipples), and everything's been fine, until today. She nursed fine all morning, but early afternoon she refused to nurse. I could get her on the breast, but even though she was going through the sucking/swallowing motions she was screaming the whole time. Once I gave up she took a bottle of pumped milk just fine. I tried again a few hours later with the same result. I'm able to pump fine and she will take the milk from the bottle, just not from me. It's safe to say this has never happened before, and it's been kind of traumatizing for me and for her

Does anyone know what could be causing this? I'm going to call my LC first thing Monday, but this has been a really upsetting day for me.
Re: 3 Weeks Old- suddenly refusing to breastfeed
Make sure that you're using paced bottle feeding when giving her the pumped milk. Check out kellymom.com for procedures. If she is getting milk from the bottle much easier than the breast, she might continue to refuse the breast.
Stay strong Momma, you can do it!
We went through something similar. I was so frustrated the first few days of bfing and the pain I was in I gave her a bottle. And then she REFUSED to nurse. I was so upset and felt like such a failure. But my nurse insisted I kept trying, and a day or 2 later she was nursing like a champ. I just offered it and if she refused I waited about 20 minutes and offered again, not offering a bottle only my breast. I did that over and over for about an hour and finally she gave in and latched. My LC told me she was just being lazy.
I also recommend trying different positions, and she could definitely be gassy. (My DD also REFUSED to burp most of the time. Lol.)
She's 17 months old now and still nursing!
Hang in there!