Could it be that my body is going through changes?

I'm 23, not ually active but have been experience some weird sh*t. Last July I got the shot, but Neva had so I didn't see the need to get the second one. I just went 8 months no period, early July my period comes, last 7 days, and then today it came again. What does that mean? I ask could it be my body changing because for the past two weeks my have been hurting and there a little bigger. I know Im not preg, so what is the problem? I just made an appt with UConn for Monday but any feedback would help til then. Only serious ppl rely please.....