Infant crying after feedings

For a week now I have had to supplement formula (Good Start Supreme) with my pumped BM to my 6 week old daughter (34 week premature). She is on Axid for reflux. She has always been a grunter and gassy. But lately she is always crying after she eats. She grunts and stretches her legs and turns red from pushing. I notice also she will grunt in her sleep and I will see bubbles coming out of her mouth. Just don’t know if this is all from the formula aggravating her reflux or from gas. Or if the meds aren’t working. Just very frustrated. So hard watching her struggle in pain. She seems so unhappy all the time.