Falling asleep while nursing

My daughter passes out every time she nurses after one boob. Not just a little, but HARD to the point where nothing gets her up. She is gaining weight, but she never does this after a bottle during the night. She's not getting a full feed (I think) when she passes out because she's up much sooner when she passes out than when after she gets a bottle. Should I try to keep her up and try and nurse more? If I can get her up, she's usually up and cranky for at least an hour after nursing then and she may not even nurse.