Baby cries while breastfeeding

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...