My son has been weird in the weight area too. I was breastfeeding until he was 3 months old and even though I had a LOT to offer, it still wasn't enough to satisfy his appetite. Then I just couldn't keep up and even though I hated to went to formula.
However, unless your doctor has said that the baby's weight is a problem, don't worry too much. Even if he says it's a problem there are always ways to correct it.
How "tall" is she? My son is short and I believe he weighed around the same when he was 4 or 5 months old. We had taken him for his checkup and he was almost 13lbs. Then a few weeks later, we had to take him to the ER and he was 11lbs.
If she seems satisfied with what you are feeding her now, maybe you could just feed her a little bit in between feedings, you know "snack" bottles of 2 ounces or something. Is she an active baby? Does she have a lot of wet and normal dirty diapers? Then I'll bet she's fine, unless her weight continues to decline.
I hope this will reassure you. My doctor told me that everyone is so used to seeing chubby babies, and not all babies are gonna be chunky monkeys. My son only weighs 18 pounds, he only gains about a pound a month, and one of his cousins who is 6 weeks older than he is weighs 23 pounds. But, he's short and active, and happy, so I don't worry.
Good luck to you!