Bleeding after sex is common. The cervix is very vascular during pregnancy and sex can irritate it. As long as baby is still moving and bleed has stopped, you should be fine. It doesn't hurt to call your Dr if you're still concerned.
Call Ur dr ASAP. Your 38 wks you can go into labor at anytime. My 1st two babies were born around 37/38 wks. Definitely call Ur dr.