Aww Kate it sounds like you are doing a fabulous job , like the othrs have said I would increase the feeds, even if it just the one's during the day to 1.5-2hours and if that isn't enough you may have to start waking her to feed 3 hourly overnight. We had to wake DD for months to feed overnight to get her to put on weight.

What was her gains like before that, and just know that some BF'd bubs do only average 80-100grams a week, DD's paed said he wanted 150, but when we saw him at 6 months he said that 80-100grams a week is fine as long as they are gaining. I wish I had seen him again earlier but I know for this time

Goodluck and well done