All day today she has nursed fine right away and then near the end of the feeding she tugs at my nipple and sputters and screams bloody murder. I thought it was gas but even if she burps and I put her back on the boob she'll do the same thing. I've tried lying down and nursing against gravity too and that didn't make a difference. Is this just a growth spurt? Is she just over tired? I don't know what's going on...