Starting to worry O_o

I'm 17 weeks today, this is my first child and I'm barley even showing. I eat like crazy so I don't get why I'm not gaining weight... I was a zero and now I'm wearing 3/5/7's depending on if I'm bloating that day or not, some days I'll be showing a cute little bump and others it's not as noticeable.... last night was the very first time I felt my baby move! I worry that maybe I didn't really feel my baby move and it could have been gas or something but I know what gas bubbles feel like and what my cramps feel like but what I felt made me speechless it's like their was a very gentle little wave/ butterfly in my belly for just the smalles second... when I tryed to get him/her to do it again their was nada happening... any ideas? Or advise???, like I said this is baby #1 for me, and I have no family to ask so all opinions welcome! Please and thank you!!,