the possible causes for your bleeding are,
-as blood supply to local organs will be increased in pregnancy
little friction can lead to spotting,
-placental causes like placenta previa
, abruption etc.
to rule out these you can get physical examination once and one ultrasound
if any abnormality gets detected in these tests you can take treatment accordingly,
better to have preventive measures like progesterone tablets
with your doctor's advice.