What eare some reasions for a shorter than normal menstral cycle?

Cycle or period?: If your most recent period was shorter than usual, it is possible you are pregnant and it was not a true period. If you are referring to your actual cycle length being short (the time from the 1st day of 1 period to the 1st day of the next), "normal" is from 21-35 days. Change in diet or exercise and stress can affect your cycle. It is a good idea to visit a doctor who can evaluate the problem.