Still spotting..

I’m 8 weeks today and I’m still spotting.. this has been going on for one week now and I’m started to get really concerned even though I’ve have 2 ultrasounds and with the last one, the doctor told me the baby looks great and has a strong heartbeat but he doesn’t know why I’m bleeding.. He didn’t see any blood in the uterus while doing the scan so he told me not to worry too much but I can’t help it especially when this hasn’t stopped and I’ve been crampy. Has this happened to anyone else before and they went on to have a healthy baby?