The coughing causes increased intra abdominal pressure.That may have lead to slight separation of the placenta. This causes some pooling of blood called sub chorionic hemorrhage. Usually small amonts resolve on its own.If your ultrasound didnt pick up anything,then it was extremely mild and not significant.But you need to be careful in the future.Dont do any heavy work and stay at rest as much as possible. You have to differentiate between vaginal and urethral bleeding. If the blood is there in urine,then thats probably due to a urine infection.

Bleedind can be due to a condition called placenta Previa,which means the placenta is too low lying and due to sudden increase in the intra abdominal pressure due to sneezing would have led to bleeding. So kindly ask your sonologist to see the position of placenta and not to worry as the placenta would migrate up if it is low as the pregnancy advances. If the baby is fine and active need not worry,just see ur doctor once,so that she would examine you once to rule out any local causes off bleeding and seek further advice from her