um it could be a few things. it probably wasnt your baby though, sorry to bust your bubble lol. your baby moving the first times feels like gas/indigestion honestly. almost like little water waves inside your belly (well uterus but belly sounds better lol). you probably felt a nerve being pinched by moving the wrong way/too quick. i get that ALL the time and this is my second, and im like 20 weeks. you should feel your baby the first time at about 18 weeks or so. its hard to tell what it is at first but once it happens more and more, you'll know EXACTLY what it is and feels like!!
congrats!! and sorry for being so blunt. i think my hormones got the best of me =]
You probably just pulled on a muscle that's already being stretched, but there's a chance it was a contraction (some women have mild contractions throughout most of their pregnancy.) My early pregnancy contractions felt like my uterus got really hard (like a softball) and tight. It was uncomfortable, but not painful.