No period in 3 months

First thing that comes to mind, although you are still young, is the early onset of menopause (it usually doesn’t begin before 45), since you say that your periods have been regular until September and that pregnancy test came negative. It is common that menstrual period generally becomes either more closely or more widely spaced as the menopause begins and levels of estrogen and progesterone drop, which would suggest other reasons in your case, but also it happens in some cases that menstrual flow suddenly stops and that sudden halt becomes a first sign of menopause. Do you feel hot flashes or experience mood swings? But, as this is not the typical case of onset of menopause, your period could have stopped because of some stressful experience, emotional problems, extreme weight change or various other serious hormonal problems. There are too many possible reasons that the only wise thing would be to see a doctor. By the way, when was the last time you’ve been to a gynecologist?