Do you watch what you eat closely?
Are you nervous when you nurse her?
Both foods you eat and if you are nervous can cause gas. Many foods cause gas. Make sure you to avoid these. Here is a website.
http://www.babycenter.com/404_are-there-any-foods-i-shoul...
I had to stop nursing at 4 months because the doctor finally said it was my nerves that was causing gas. I never understood because I did not feel nervous but a few years ago I was diagnosed with anxiety and now take medication. It was probably there years ago but not to the extent it is now.
Another thing you can do is rub her stomach or back while nursing. It may help her pass the gas easier.
Good Luck