Why haven't I started my period?

I'm late to start, my due date was three days ago, I usually get cramps a day or two before it's due to start but so far there's been no sign of anything

I was starting to worry I might be pregnant even though I was 99% sure I couldn't be, my boyfriend and I are always safe, so I even took a test today but it came up negative (although did I take the test too soon for it to come up positive if I am?)

What other reasons can there be for it to be late? I've only rather recently lost my virginity and started a sexual relationship with someone could that have something to do with it possibly?

I have no idea if I'm regular or not because I never keep track of it but I've always known when it's about to happen and I definitely know it's late