What Causes Vaginal Bleeding After Finishing A Period?

Hi my qeustion is about miscarriages i had normal cycle for 5 days than 1week later i began another cycle, i didnt think about it until the fourth day of cramping and such that i went to the hospital they did a pelvic exam and preg. test(urine) which was neg. they said they didnt know what it was to follow up with my doctor i am going to see him but its bothering me not knowing could it have been a miscarriage