What could be the cause of 3 months of missed periods if you had a tubal ligation 2 years ago and 2 pregnancy tests within the last 2 months were negative?

Periods can be missed for a variety of reasons besides pregnancy. Some women just have irregular periods. Extreme stress can cause periods to temporarily stop. Women who have very low body fat like ballerinas often stop menstruating. Female athletes, especially those who rigorously train for long-distance events like triathlons or marathons often stop menstruating. Menopause or peri-menopause can begin as early as your 20s and cause light or missed periods. Either way, if your periods have been regular in the past and now you've missed three, I would see an ob/gyn.