Breast to bottle

My baby girl initially didn't want to breastfeed because my boobs were engorged...so i started offering expressed milk in a bottle. She was perfrctly fine. In the meanwhile i kept offering my boobs...she soob began breastfeeding. My mother and mother in law have babysat in the past feeding her theough bottle. I will be going back to work in a week and milk supply is getting low. I am trying to bottle feed frozen expressed milk and/or formula but she is refusing the bottle.... help !!! Any suggestions or any experiences you have guys have had ???