Should I be worried about this?

Its alot more common than you think. Rocking back and forth does not mean your crazy, or autistic. Its a way for you to soothe yourself. Remember how a mother rocks a baby and how usually it calms them down. Well, in short, your self-soothing. No, your not a baby, lol...but its a way that your body/mind has found to kinda calm you down. I do it alot when i read a book. And can catch myself setting at the computer gently rocking ever so slightly. Its really no big deal. Try not to think of it that way, be thankful that its actually pretty healthy! If you feel self concious about it when other people are around, just bob your head or tap your foot with it like your thinking about a song, people will think that your doing that and just "chillin" out. :)