Time to see a gynecologist! Going more than 3 months without a period is dangerous because of unchecked uterine lining thickening. They’ll need to prescribe a course of progestin to induce a bleed. If this happens a lot, ask your doctor to look into the cause. It could be PCOS, thyroid issues, or a number of other factors.
I’m sure it’s already been offered at this point, but you should seriously consider getting on birth control both for your immediate health and to protect your future fertility. It will keep your uterine lining thin, so you don’t have to worry about developing uterine cancer, and can protect your ovaries. It’s a catch-all treatment, not just a bandaid.