Let me tell you that - BREASTMILK IS ALWAYS THE BEST FOR YOUR BABY.
It would be better if you could store the expressed breast milk and feed your baby. However, you should take care that milk is properly collected,stored and fed.
Her weight is ok according to her age.
Please check that-
1. Formula milk is properly prepared. Preparation must be done according to the recommendation. Usually, 1 scoop in 30 ml water. Diluted formula may be the cause behind not gaining weight. Don't allow to give her plain water, this will aquire volume of milk that could provide nutrition.
2. Make sure that your baby is fed on demand. Don't restrict to any strict schedule. Make sure that she is fed at least 3 hourly. For her weight, 70-90 ml every feed would be adequate.Remember, It will increase as the baby grows.
3. Keep your baby properly covered to keep her warm. Hypothermia
could be another cause behind decreased weight gain.
As you are working mother, make sure that your babysitter takes care of these.
If still her weight is not increased, please visit your pediatrician
Hope I could make it clear. Please post if you have more queries.