ANDY'S ANSWER: Puberty is a process that takes several years. The changing hormones in your body affect a lot of things, like body odour, hair growth, breast development, mood swings and somewhere in the middle of it all, menstruation.
Cramping can be a symptom that your period is about to come. It sounds like you are at the right stage of puberty to be expecting it just about any time. Carry a panti-liner or pad with you at all times so that you are prepared when it does arrive.