Sounds like she's doing fine to me, they all put on weight at different rates and as long as she remains alert and active has wet nappies daily and poos at least once a week then try not to be concerned.
this page from kellymom.com might reassure you. I agree with QoQ, doesn't sound like there's anything to be concerned about - but kellymom.com is a great website for breastfeeding mums especially if the health professionals around you are less than encouraging.
lockets, mine is just the same - birth weight 7lb12, probs establishing bf, now 10lb2oz at 14 weeks. After a whole lot of worry I've now decided to calm down, he's gaining weight and is perfectly cheerful, just a bit slim. nice hv says he'll probably catch up when he starts eating plenty of solids, nasty one says give him a bottle - guess which one i listen to
Can I recommend not weighing? I don' t know what my dd weighs as she's only been weighed twice in her life - once at birth and then before her jabs last time. I'm exclusively breastfeeding her and she's a week off six months and I think she's looking a bit slimmer these days but she's clearly fine - active, happy, smiley, weeing and pooing etc - and I find not having an HV on my back and no scales to torment me mean I am so much more relaxed.