I wouldn't stress,

it is quite plausible that bubs is gaining, while you are loosing in other areas,
check with your doc/midwife,

definetly agree on the calcium though, make sure you are getting enough, because that is the hardest thing to get back, once a women reaches her early to mid 20's you stop creating calcium stores, and can only maintain them.

with DS, I weighed 63kg's at conception & 78kgd just before he was born (I gained 15kg).

this time I started off heavier at 73kgs, and am currently only 80 kg's (only gaining 7kg) - after eating easter eggs and all, i am looking forward to loosing the water, baby placenta & excess blood weight in the first few weeks, I might be skinnier than I was pre conception!

although was a lot sicker this time also, and found i could not eat at night. Found it easier to have a big lunch like pasta and a light dinner, bowl of veg or even just fruit.