This actually happened with my first pregnancy because I was so sick, and I never gained it back. (DS still weighed 8lb13oz, even being born at 38 weeks lol.)
It happened again this pregnancy--I wasn't nearly as sick, I just had no appetite. All food sounded completely gross...until about 2 weeks ago when I hit 25 weeks. I feel like I eat nonstop. I guess it is all going to baby, because I still haven't gained any weight back.
I did drink some protein type shakes (Boost) to help fill in the gaps before I started eating more. If you're worried about it, the shakes might not be a bad idea. My doctor basically told me that baby will take what he needs--so if I don't eat enough, I will be the one lacking, not baby.
I'm sure everything will be fine with your little one! But just in case, like Mushy said, fill up on what you can, and get the extra nutrition from the shakes if you have to. You might hit an appetite spurt like I have, also. : )