While a 21 day cycle is still considered to be within the range of normal, the combination of cycle change and now a "too-light" flow MIGHT suggest a change in ovulation pattern. The most common reason for a menstrual pattern like you have described is either a change in ovulations--or not having an ovulation every month. In a normal cycle, estrogen is produced all month. Estrogen is responsible for building up the lining of your uterus so you have something to shed each month.
In a normal cycle, progesterone production increases following ovulation and release of an egg.. Progesterone "stabilizes" the uterine lining in preparation for a possible implantation of a new pregnancy. If you are not pregnant that month the levels of estrogen and progesterone fall, triggering the release of the uterine lining—your period.
So, if you do not ovulate, the estrogen build up of the lining continues, but without the usual ovulation associated progesterone. Thus, the hormone levels don't decline, and the lining stays up inside the uterus—as a missed/late period. Alternatively, the built up lining can begin to shed on its own creating erratic bleeding patterns which are usually "too-light" or super heavy and prolonged.
There can be many causes for ovulation changes: thyroid problems, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with boyfriends/girlfriends, finances), increased body weight, anorexia, rotating shifts at work, etc. One of the more "normal" causes would be advancing age. Cycles begin shortening as we move into our 40's.
If your cycles continue to shorten and/or persist in their "too-light" nature you should see your GYN or clinic. They may want to do some blood work (eg prolactin level, or an FSH [menopause> test depending upon your age and family history).
Yours,
Jane
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