Are you certain the spit-up is from overeating? I'd maybe try pumping and feeding with a bottle first to see if that's really the case... if not then its possible she's not liking something youre eating which could be causing an upset tummy. Both my kids were small eaters and never nursed on both sides at a time, I would just pump whichever side they didn't use for that feeding and save it.
You might be eating something that her body can't handle. Most common thing would be dairy. I'd stop eating or drinking anything with dairy to see if that helps..
Um try burping her every 5 minutes sometimes I just get too much air in the stomach
It could be other things to like the foods you are eating she could be allergic to something that you putting in so try new things to eat and see if that works.