Usually this happens in your late 30's, early
It can also cause a whole variety of symptoms
including hot flashes, breast tenderness, weight gain, fatigue, decreased sex drive, mood swings, and of course irregular period.
Did your doctor check your FSH levels, as well as your estradiol levels. You can request a history of all your testing to see if these were done. If not, you can request they do them. If you have had it done, your levels will be indicative of what might be going on.