Contact bleeding
The cervix is richly supplied with blood during pregnancy. A small bleeding after sex is caused by the touch of the penis against the cervix and in most cases stops quite quickly. Sometimes it's a little bit of blood and sometimes it seems quite a lot. It may also take some time for the bleeding to stop.
After a while some old, brown blood can come out and that should be the end of it. If this is not the case, please contact the midwife.