No period

So I used to be vegan and i would never get a period. Barely anything at all sometimes. And now that my diet is back to normal i will have it one month and it doesnt happen for another two months or so. Can someone tell me why it does that? I mean i have had it over 4 years now it should be regular and every month. The first few years it was fine . Now i just dont get one. My boyfriend is afraid i wont be able to get pregnant due to this.