missed period for 3 months, neg pregnant test.

The word for not having periods (often when you should be having them) is amenorrhea. This can be caused by many things. Pregnancy is just one, along with breastfeeding and the menopause. If you have a missed period and negative pregnancy test, you're really unlikely to be pregnant anyway, especially because it has been so long. Some medications will make your periods stop, as well as stress, underweight, excessive exercising, a pituitary tumor, PCOS, thyroid problems, and more. See a doctor to figure out what is going on. If it's something like PCOS or thryoid, you will need treatment and you can't access that without a diagnosis.