Help me please!

I am having trouble with nursing, and need some advice.  For the past few days James has been pulling off and crying and acting like he isn't getting anything after a few minutes on one side. I will switch him to the other side, and he will do the same thing. So then I will go back to the original side and start over. Some feedings he will seem happy after getting each boob twice, and other times he still seems hungry. I can't figure out why he is doing this to me.  Has anyone experienced this problem?