@jennythelib - The answer's right in front of you! It's almost impossible that you are really having such a short menstrual cycle. The "short" periods you're noticing are probably actually ovulation bleeding! You ovulate, then a couple of weeks later, your period begins as expected. Spotting around the time of ovulation is actually an indicator of good fertility, so that's good news for you!
Personally, I had long, irregular menstrual cycles when I was trying to get pregnant. I ovulate late, and that was often the reason, but sometimes I wouldn't ovulate at all and my period just stayed away. When that happened, I called my doctor and she prescribed progesterone to start my period. (She made me take a pregnancy test first just to be safe, even though I knew I hadn't ovulated.)
Eventually, my cycles started going again on their own. One more thing to try for anyone concerned about a long menstrual cycle and fertility!