I've just looked at my girl's growth charts and my LO was 7.5pounds at 2 months, born at 5.5pounds. She was considered underweight because she was below all the lines on the weight chart.
Looking at the charts we get given, your LO's weight is between the 2nd and 9th centile. This means they are within a "normal" range, smaller than a lot of the population but not underweight. However the actual centile line isn't always important. What should be looked at is whether she is in proportion for her length and also whether she is growing at a steady rate. e.g if she was 9.5 pounds a month ago and between then and now hadn't put on any weight, that would be concerning. If she started had been on the 50th centile till 2 weeks ago and then suddenly dropped to the 9th, that could be concerning. If she is growing and looks in proportion, then I'd say that weight is fine!
It is very rare for women to not make enough milk when their baby is latched correctly, the mother is well nourished and they are feeding on demand. Unless you have concerns over the latch (is it painful? do your nipples come out a funny shape after feeds? are they cracked? do you get repeated bouts of mastitis?) the top-ups could actually reduce the amount of milk you produce. I started top-ups at 6 weeks because of poor latch and low weight and had to use a breast pump every time I gave her any formula. This was exhausting and time consuming but I knew if I didn't do it I might never make enough milk for her.
I'd ask your doctor on what basis they say she is underweight and if you still feel you are not making enough milk you need to start pumping whenever she takes a bottle. Night feeds are the best at increasing supply so maybe swap that night bottle to a daytime one and breastfeed at night. BUT only if you really are concerned about supply. From what I can tell what you are doing sounds like it is working for you and it seems insane to start killing yourself pumping and topping up based on that weight without additional reasons to be concerned.