Spit ups only from nursing

Ok so we have had major issues with my 12 week old daughter concerning spit ups, since she was about 4 weeks old... long story short- I am dairy and soy free now, and she's on Zantac 3x a day to alleviate pain from the reflux. This week I returned to work so I'm pumping and she's being bottle fed. Suddenly, she is no longer spitting up (per my fiancé and mom.) however, as soon as I breastfeed she spits up again. I'm so confused and am now wondering if this is actually a breastfeeding related issue (Fast letdown maybe?) anyone have any ideas??