Bad pumper

My sixth month old is exclusively breast fed, aside from supplementing as a new born until my supply was better. Anyway, I have awful results with pumping usually. I don't get much at all and she seems less satisfied at her next feeding so I always feed from the breast. My anniversary is coming up and I would like to go to dinner with my husband. Would a feeding or two of formula be hard on her tummy if she isn't used to it?