Breastfeeding and formula?

So my little girl is a month old now and nursing just hasn't worked out for us but I still wanted her to get the nutrients in the breast milk so I was trying to exclusively pump. I wasn't able to keep up when her first growth spurt hit. So I gave her the honest brand organic formula. Is it okay for me to still keep giving her breast milk a couple times a day also or will it upset her tummy to have both?