Bleeding early. 20-30% of early pregnancies will have some bleeding in the first 13wks. Uterine cramping, (like a period), and a size less than dates measurement on ultrasound are also worrisome.
Risk of miscarriage. A diagnosis of "threatened abortion" indicates that you've experienced cramping/bleeding during pregnancy that raise concern for possible miscarriage, however your exam shows a closed cervix and a fetal heartbeat indicating that things might still be just fine. Threatened abortions can resolve and normal, healthy pregnancies result afterwards. Be seen by your OB for any further bleeding/pain.