My baby crys after nuring for only 5 minutes...What could it be???

I have a 2 month old baby girl. every time shes hungry I nurse her as long as she wants, but about 3 out of every 5 feedings she will only eat for a few minutes and wiggle her head side to side and fuss and then cry. I try to change her diaper, make sure she is warm and give her a pacifyer, but she still will only suck like 2 or 3 times and start to cry. Its like im not making enough milk for her. she is a big eater, and I'm concerned shes not getting enough. She also rarely sleeps during the day and at night it takes alot of patience to get her to go to sleep. When she is tired, however, she fights it...She cries and doses of, cries, spits out her pacifier...and does this many times. Anyone have any advice???New mom here :)