What Is The Cause Of A Delayed Period?

Hello, my period is 8 days late. For about a week now, I've experienced period-like cramps, I have a pimple on my forehead which I usually get before my period is supposed to arrive, I have also been burping a lot which has me worried since I read that it is a sign of early pregnancy. My breasts are not tender, I do not have any cravings and I do not urinate a lot. I just took a home pregnancy test and it came out negative. I don't know if it makes a difference but I usually had 28day cycle and for the previous two months my period arrived early, which made me have a 25day cycle. I track my periods and thought that I understood when my fertile days were and when I ovulated; during this time my fiance and I do not engage in sexual intercourse. I cannot focus on anything else except thinking that I might be pregnant, we are definitely not ready to have a child at this stage in our lives.