9 week old daughter sleeping more, not nursing as much?

at her 2 month check a couple days ago the dr said she was gaining weight well. she has had a cold and has been congested, and she seems like she has been sleeping alot more than usual. not nursing as long as she used to. when she is awake, she seems happy though. smiley and everything. she will latch on fine and start to drink, but she falls asleep within a few minutes. i wake her up and she will then take the other but be kinda squirmy. i think my milk supply is decreasing, because she is not nursing as often as she was before. i used to wake up really full, and im not any more. please give me some advice. besides pumping. when i try to pump i get nothing but sore . thanks