I feed but just in case i need to use formula?

Well when I had to start weaning my 8 1/2 month old (he's now almost 10 months) he would absolutely not take anything other than parent's choice gentle formula. I wouldn't suggest switching back and forth between formula and feeding. It could make her tummy upset, as you've already experienced. milk is best so I would just continue doing that instead of worrying about formula. Just pump when you have to be away.