My ultrasound today and showed a blood clot in my uterus. my period is tomorrow. can they tell if that is your period starting? pregnancy test was neg

Pelvic ultrasound: Ultrasound cannot predict when your period will start. If there was something seen inside the uterus right before an expected cycle, it could be a blood clot. It could also be something else. What was seen could certainly be blood or debris indicating the beginning of a period, but I would encourage follow up to be certain that something else is not going on inside your uterus.