Breastfeeding question

My baby is 12 days old. I’ve been breastfeeding but having to supplement with formula bc she isn’t gaining enough. Anyway today I saw a lactation consultant because I needed some help. She said because I’m so stressed to see if i wanted to try pumping exclusively (I got mastitis the other day in the left breast). So I said I would try it. I got like 2oz out of each breast this morning at my appt. with her. She said to pump every 3 hours and give her 3oz as she should be gaining .5 oz a day. Well here I am pumping and I only got 0.5 oz out of the right breast and nothing out of the left (for 15 min). Is the fact that I didn’t nurse the reason for the lack of what’s coming out??? I was just listening to her advice as a LC and thought she knew what she was talking about in saying there was no harm in exclusively pumping. Anyone only pump and have success?