Congratulations! I really wouldn't worry too much about weight gain or otherwise. A long as you're healthy and your midwife is happy it's not really a reliable or useful measure and there are lots if definitions of 'normal'. From what I've read on here you sound on the low side of average. I'm 29 weeks and have out on about a stone and a half. Have some cake!
I always worried about this, I was ill in the first 3months and gained nothing. I had people constantly telling me I was tiny and making remarks. 12wk scan was fine, 20wk scan showed baby as slightly big for dates and growing well and a scan I had atabout 32wks showed baby reasonably big for dates. I think last time I weighed myself I'd put on about 1.5stone at 28wks but I was prob classed as borderline normal bmi to start at about 8stone and 5ft3" As long as you're eating regularly and growing I wouldn't worry. Good luck.
7lbs at 21 weeks sounds fine. Like others have said, don't worry about it. Your midwife will raise anything she thinks is a problem. At 21 weeks I was probably the same as you, I'm now 30 weeks and have put on just over a stone.