Pumping while supplementing?

satteberry

My LO will be 3 months old on Monday and we've been EBF.  We went in for a weight check on Friday and since she'd only gained 5 ounces in three weeks, her pedi told me to continue to nurse, but follow-up with 2 ounces of formula to supplement after each feeding.  LO only eats off one side for about 20 minutes each feeding.  How/when do I pump in order to keep my supply up, but still have enough for her to eat?  Do I pump both sides after she eats?  I know my supply has dropped and I just want to try as best I can to give her BM for as long as possible.  thanks for your advice!