Missed period? Not had sex.

Hey ladies, I dunno if someone can help me in this department.

I’ve never missed a period since I started my period and I was 14 when I first started, I’m now 21. I’ve been pregnant once and that’s the only time I was late.

I’m currently not in a relationship and not having sex (haven’t for 3 months) and my periods late by 7 days. I’m not pregnant because I’ve had periods since then, I.e - last month.

I’ve been getting niggly pains etc but I’m just very confused why I’m late, does this happen when you grow up, does your cycle change?