sounds like you have had a growth spurt and it sounds pretty normal to me... as you sound like you are v healthy with exercise and diet i wouldnt worry!!! I think weight gain varies by person, so dont compare yourself too much to what books say or what other say - as long as you are eating well, keeping reasonably active etc it sounds good to me!
I know that we all worry about these things though so you could always ask midwife next time, not to weigh you but just for peace of mind. Thats what I would do.
Has your tummy grown a bit? I imagine the baby would be having some growth spurts too.... which is a great reason to pile on some weight!!