Blood clot??

I'm 7 weeks pregnant and I had my second ultrasound today, when they did the ultrasound it showed the baby was growing as normal and the heartbeat was healthy. However, the ultrasound also showed that there was a blood clot in my uterus.. This really has me freaking out, the nurse said it should go away but there is a chance it won't leading to miscarriage. Has anyone else been through this or had this happen??