Missed period

[Kn33]

Earlier in the month I had some spotting, which is unusual for me, I think it was too early in my cycle to be implantation bleeding or anything like that. I've had weird pains all month, and am now 5 days late. Multiple pregnancy tests all negative. My question is what else could be going on?! My periods are like clockwork normally, I'm not stressed, haven't been ill no diet changes or weight changes or anything I can think off that would affect my cycle. We're not TTC so not desperate to be pregnant just now worrying why else my period could be AWOL