I'm sorry you're having so many problems!
Indeed gas (and constipation) can cause severe pain. Folks sometimes think they're having a heart attack, it is so painful. It's good that you got it checked out to be sure it wasn't something more threatening.
If the Avonex and rebif -- both interferons -- are causing you such trouble, have you (or your doctor) considered Copaxone? Most folks find it relatively trouble-free of side effects, and might be a good choice for you.
I hope you feel better soon.