Help!

I haven’t had a period for 90 days. Period was pretty normal before then. Went to the doctor. I am not pregnant, yes we’ve been trying. Blood work came back great with no issues. Went for an ultrasound, also came back great with no issues not even PCOS. I’m at my wits end, and I don’t know what to do to bring it back. It’s probably stress, maybe some weight gain. I had a friend who’s a nurse suggest I eat very very clean, and try taking prenatals to help regulate my cycle. Any other help would be appreciated.