My daughter did the same, I was told by my lactation consultant to gently rub her bottom lip to get her to suck longer, or rub her cheek, her ear... But if all that fails which it did for me, I second Ali, feed often, as much as LO wants or needs, I fed my DD every time she cried and to be honest even when she didn't because I needed to boost my milk supply. Your LO is so young all you can do is make her only source of nutrient available to her often, very often and if she gets enough opportunity and if you can increase the time she stay awake on the breast by even 10% by rubbing her gently, that times many many feeds, chances are she will be getting what she needs and it will keep your supply up for when she is more active and awake. Good luck and keep up the good work mama!