Late period, help.

My period cycle is usually 28 29 days on the dot. The last time it was late was before I graduated college a year ago and I was super stressed from finals. I have not been stressed and I’m healthy and everything so I don’t know why I’m on cycle day 36 with no period? Or any period cramps, which I get before my period. Took a equate pregnancy test today and it was negative. What other reasons are there for being late? Oh and yes I did have unprotected sex and I’m hoping my ovulation timing was right which would make my chances of not being pregnant pretty accurate. But I just don’t know 😩