Ideas? 😳

When I was 13 is when I FIRST started my period, I'm now 18, but for two years (from 16-18) I had stopped getting it altogether. My doctor said it was most likely from being underweight. 
Anyway, I started getting them again after my boyfriend and I had sex for the first time. 
He wasn't my first, but he's rather large... 
Could having sex with him have started my cycle up again or what?