Dear An: I agree that neither the fibroid, nor the overly thickened lining, are likely causes of your missed flow. If a 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. You apparently have enough estrogen because the ultrasound revealed a thickening of your uterine lining.
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(s).
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.
There can be MANY causes for not ovulating: low thyroid, pituitary problems, ovarian cysts, physical stressors (eg sudden increases in exercise, crash dieting), emotional stressors (problems with parents or spouse or kids, finances), increased body weight, anorexia, rotating shifts at work, etc. In your specific case an ovarian cyst is less likely as it would have been noted on ultrasound. However by our 40's (due to "aging eggs") missed ovulations become more common--with the most frequent missed ovulations happening during perimenopause (ie age 47-51).
There is a second, less common cause for not having a period. In some women the estrogen levels become very low. The levels are so low that no uterine lining is available to be shed. When these women are given the progesterone pills to take, no flow occurs after the medication is finished. This tells the GYN that the woman is not making enough estrogen to produce a thickened uterine lining.
It sounds like your GYN care has been very thorough with blood work, imaging, and a possible follow up based upon your ultrasound. When you go back for your follow up visit you can ask specifically about missed ovulations. They will be able to give you the most "for sure" answer.
Yours,
Jane