Is my babies suckle not effective during feeding?

During letdown my baby has no trouble. She gets the milk easily, but sometimes she pulls off of my and acts very upset like she's not getting anything and then she'll latch back on and have good suction for a minute and then pull back off upset again. We can't really continue to feed this way so I will give her a pacifier and pump. This morning I was able to pump 5 oz after she started doing this. She had already nursed for 10-15 on the first and had been on the second for several minutes also when this started. Any suggestions as to what is going on?