Hi, I was exactly the same until about 14 weeks, and I still have days now where my appetite just dies (25+2 now). At my booking in appointment they had me down as overweight too. Since then I've put on the grand total of four pounds, because I lost a good bit those first few weeks, and there are still some foods I simply cannot face. Salad, and prawns both turn my stomach, oddly enough.
Anyway, my baby comes up absolutely fine on all the scans and measurements, and they think bub is on track to be about 8 pounds based on the 20 week scan. Every time I say to the midwife I haven't gained any weight she just says baby is fine, blood pressure is fine, if you're feeling ok, don't worry about it, just make sure you're getting plenty of fluids. So I don't worry too much!
Just eat what you can, when you feel like it. I had a lot of mcdonalds milkshakes in those first few weeks, and tubs of fruit. Ginger biscuits also helped with the nausea. I went off fish totally, bbq meat made me heave a couple of times, and anything remotely stodgy was a no. But now I have days when I can eat for england, I'm eating breakfast every day for the first time sonce I was 10 years old, and god help a chocolate bar if it wanders into my field of vision! The lack of appetite will pass for you too, I'm sure.
Good luck and congrats :)