If you get your period already, you may be close to that time of the month. If youre sexually active you could be pregnant ( i hope not miss 12 year old!! ;) otherwise like above stated...ur breasts could be growing...it hurts alot when they do!!This happened to me last year. I didn't know what it was and still don't but it went away so i put it down to being my period coming that month. It was very painful. Not the same as growing pains at all. Very sensitive.