Weight Gain Question

I had the DS in November 2011.  I'm starting to obsess and worry about my weight.  At 12 weeks pregnant  I haven't gained any weight, in fact I've lost 5lbs.  Wouldn't be a big deal but I'm already at a normal BMI so now I'm worrying I'm not absorbing enough to get my baby what it needs.  Anyone else have problems gaining weight?  Am I just obsessing over nothing?  I go to the OB this week and down to see my surgeon in a couple of weeks but just thought I would see what y'all had to offer.

I had trouble gaining before I got pregnant, but am gaining fine now- 20 weeks. I am in a group with other ladies, and they are all having a really hard time gaining, but all their babies seem to be growing fine!!!!