D.C.
Could be related to your overproduction problem. How are you handling that?
I had the same issue, and I used a hand-pump before feeding the baby. I would do just 20 pumps with the hand pump to get rid of the most forceful letdown (which is when the baby can swallow a lot of air trying to keep up) then let him nurse. And I would then only have him nurse on 1 side for the entire nursing session. Nursing only on one side lets the baby get plenty of the rich hindmilk, and also helps to decrease your supply in the long run.
Another entirely different possibility is that your baby has an intolerance that bothers her tummy after nursing. Try cutting dairy out of your diet for 2 weeks and see if she's less gassy and spits up less.