Dear An: Congrats on your new baby! If you have any possible risk of pregnancy, that is the first condition to be assessed. Even reliable contraceptive methods such as birth control pills, and tubal ligations can fail. So it is important to get a pregnancy test done first--which you have done.
If the pregnancy test is negative then the next most common cause of a missed period is not having ovulated that cycle. 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. 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—your missed period.
If you have been several months without a period, a gynecologist may give you some progesterone in a pill form (eg Provera 10 mg for 5 days). Within 48-72 hours after stopping the progesterone your "progesterone blood level" will fall, triggering the release of the lining that has been building up. Many women report that these periods are very heavy-- as though several months of lining are shed.
Causes for not ovulating are NUMEROUS: thyroid problems, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with spouse, boyfriends/girlfriends, finances), increased body weight, anorexia, rotating shifts at work, etc.
If your regular cycles do not return by next month, or if you develop other symptoms (eg pelvic pain from an ovarian cyst or increasing pregnancy-type symptoms) contact your OB/GYN for an evaluation.
Yours,
Jane