Period late by 6 days

My period is always very regular and a week before I always get normal PMS symptoms like sore breasts, acne, cramping etc. This month - NOTHING and now my period is late by 6 days and it doesn't feel like it's on it's way soon either.

My SO and I are not TTC so we are generally very careful. We haven't used condoms but as soon as I climax we stop having sex and I finish him off with a blowjob. I've read that precum can contain very faint amounts of sperm but we've been having sex for 2 years without condoms (without him finishing in me) and we've only had one pregnancy scare.

What could be the cause of my missed period?! I haven't tested yet because I guess I don't have any suspicion that I might be pregnant (well I hope I'm not).