How do you know the difference between an overactive letdown and just plain old gas?

My dd is 7 weeks old and I've thought she has gas because after about 10 minutes on the breast she starts sputtering, popping on and off, crying, etc. She will go on and off frantically after a little bit every 30 seconds or so.

Since it occurs about 10 minutes after she started I'm assuming its a belly full of foremilk gas causing the upset.

Am I right? What else could it be?

Because of this on and off pattern, I don't think she's getting enough of the hindemilk (her poops are seedy greenish yellow half of the time and yellow the other half - no seeds).

Any thoughts? Any advice?