FORMULA AND BREAST MILK

hello there,
sorry to write in English, i can read Hebrew though.

i have 11 weeks old baby girl
2 days ago i have started to give her formula once a day (S-26- the name of formula) and i also top it up with breastfeeding during the day and night

my question is:
is it possible that the baby will gain lots of weight because of eating formula?
i read that thas what happen when you give breast milk and formula

i give her formula cause in a weeks time i will be going to school twice a week and unfortunately i can not pump enough breast milk in order to save some for the time i will be away
everything that i pump my baby is eating so i have non left in the fridge