Not gaining weight

I'm 14 weeks today and can't seem to gain any weight. I drank a lot of soda before I got pregnant and gave that up whenever I found out. I also gave up fast food. I have lost a total of 18 pounds just by giving that up. Which is normal because every summer I give up soda and lose 15 pounds within a month. But I can't gain any weight now. I can't even tell I'm pregnant. Anyone else have issues like this?