hi,
there may be possibility of local
injury which might have led to bleeding as in
pregnancy there will be increased blood supply to local organs,
placental causes or vessels of membranes covering the cervical os can also be causes for this,
if your doctors preformed
ultrasound and told you that everything is good then no need to worry, if not you once undergo ultrasound to know the cause of the bleeding and continue
progesterone injections or tablets to stop contractions and to control the bleeding.
take care.