Nursing woes

This breastfeeding thing can be so difficult sometimes. I know a lot of people have trouble getting it established, but once it's established it is pretty much a breeze (well at least it was for me with Matthew, and has been from day one with Scarlett). That's what has me so dumbfounded today. For the last 2 nights Scarlett has absolutely refused the breast. I get her in position to nurse and she starts screaming. She ate fine during the day, but Friday night around 7:30 she acted hungry so I tried to feed her and she wanted nothing to do with it. I didn't worry too much at that point, I put her down to play and tried again an hour later. Again, she screamed when I tried to nurse her. I tried a couple of more times until around 11pm and finally decided to just pump and go to bed. The last time she had eaten was at 4:30. She slept all night! I couldn't believe it. I didn't sleep well at all, listening for her breathing since I was sure something must be wrong for her not to want to eat!

Fast forward to last night, and the same thing happened. She ate around 4:45 yesterday afternoon and then wouldn't eat again - screamed every time I tried to feed her. David gave her a 4 oz bottle before we went to bed, just so I could relax a little that she had something in her tummy. She took the bottle, but I know the mechanics of feeding a bottle kind of force the baby to eat whether they want to or not.

I have no idea what is causing this - if anyone has any clues, please share!