It was the cervix being irritated. More blood flow to that area during pregnancy can cause the cervix to bleed a little when it is irritated.
What did you get the shot of antibiotics for though? That seems a little like something more was going on...
As far as the placenta goes: there is no risk of it getting 'poked' during sex or the doctor even poking it, for that matter.
If you are 4-5 weeks pregnant, some break thru bleeding in early pregnancy is common.