17 weeks and haven't gained anything?!

I'm 17 weeks along tomorrow and I haven't gained any weight at all (I've barely even gained back what I lost the first trimester). I've been eating extra, and I'm incredibly healthy and take care of myself, but the most I have for a bump is occasionally looking a little bloated. Unfortunately, my next doctor's appointment isn't for another month, and I'm really stressed now! Is it normal or should I be worried?