How to tell the difference between an abnormal period and a miscarriage if you do not know you're pregnant?

I knew the difference... Started my period when i was 10,when i turned 13 some horrible things happened to me,i was so stressed my periods became irregular,i'm lucky if i get one a year now... I am now 19,things went wrong with the protection,but niether of use knew,didnt know i was pregnant,but one day i had really bad cramps,worse than usual,the smell was vile,i was wearing 4 pads and a tampon at a time,and had to change every 10-20 minutes,then long flesh like bits,about 3 inchs long was coming out,the smell just got worse and worse,i ended up in the hospital in the end and they confirmed it all... But sometimes you need to be careful,as signs of sti's can cause confusion in it all. But if yours is anything how i explainend,i would go to the doctor.

There might not be any difference. I've had a chemical pregnancy and if I wouldn't have taken a test super early I wouldn't have known, it was pretty much a normal period with slightly worse cramps. I also have irregular periods, OPKs made it a little easier to figure out when to test.