Hi! Menstrual cycles are measured from the starting day of the period to the starting day of the next period. Its always good to actually write them down on the calendar, because they may seem abnormal but on closer inspection turn out to be OK. Anything from 21-31 days from start to start is considered withing normal range, and most doctors are not going to do too much if the periods are following this pattern.
If they are getting closer together than 21 days, you might be considered appropriate for a biopsy of the uterine lining. Usually the cut off age for biopsy is 35 to 40, depending on other risk factors such as weight.
You might also be considered for biopsy if you are having bleeding between your periods, or very heavy periods, or periods that last more than 7 days. A transvaginal ultrasound can also be used to check the thickness of the lining, although this is much more useful in postmenopausal women.
If you don't meet any of the above criteria, we usually chalk it up to hormones.
What we do about the problem again depends upon your age and wishes for fertility and could include anything from inducing ovulation to hysterectomy!
Hope this helps!