Why do I only get my period after having sex? Infertile?

Okay so my period is always off and on. And this may sound strange but I think it’s linked to me having sex. I had sex with my boyfriend in October (late October) and then had my period about a week later in early November (my period, when it does come, it comes in the beginning of the month). I don’t even remember my last period before that. He went back to his hometown and I didn’t have my period after that. When he came back in January, we had sex, and I got my period in the beginning of February. I masturbate frequently with a dildo and it never “triggers” my period (I don’t do it for that cause, I’m just saying) and this pattern has been this way for quite some time now (before October and January). Are there any reported cases of this kind of thing happening? Sex causing your period? And if so…does anyone know if this means you’re somewhat infertile or even more fertile?? I read some other questions about girls getting their period early or something after having sex…but no. I don’t get mine early. It comes on time, but only the month after I have sex. If I didn’t have sex one month, I won’t get my period the next. =/