Early miscarriage?

I don’t mean to sound stupid as I’ve never been through this before. My period is 5 days late and I’m 17 dpo. I tested negative on all of my urine tests. My “period” started today but it didn’t feel like a regular period. It was very heavy nonetheless but I was in a lot of pain and having severe back pains not as usual. Around 3 pm today I was riding in the car with my fiancé and felt dizzy. I passed out! I’ve been passing these all day and I just sat down on the toilet and there was a huge one I passed that I couldn’t get a picture of. Are these just blood clots or am I having an early miscarriage? Going to my doctor in the morning.