Hi hun, I had short cycles as well, 21 days. This is considered in the normal range, however, I ovulate too late for my luteal phase to be adequate. So, I started vitex capsules in June. It pushed back my ovulation date by about four days, a good thing for me. I've had no negative side effects, so I would certainly recommend.
One thing I would recommend also is a natural progesterone cream, or some kind of progesterone supplement if you are interested in seeing a doctor. I was so skeptical of the cream (I figured since it was just a cream, it would never work), but I picked up a tube last month and it lengthened my luteal phase by three days during the first month of use. I thought this was promising. Then I happened to be in for my annual obgyn appt and mentioned my short lp to my doctor, and she gave me prometrium, a natural progesterone supplement that is safe during pregnancy. I'm on my first month of it, but my cycle is already longer than it would be without the prometrium or the cream.
Good luck to you!