Bad cramp in the uterus area

I ve had cramps on and off the past week. A few minutes ago there was a bad cramp around my uterus it lasted a good 30 seconds then went away. I'm 10dpo. Negatives pregnancy tests. I'm due for af in just a few days. Normally when af is due to show ill have spotting a few days before af arrival. But there's been nothing not a single spot at all. Why was there a horrible pain in my uterus?