See regarding chances of you being pregnant with the ejaculate outside the vagina is very very minimal,specially if you have cleaned it immediately.
When you have already taken an ipill
the chances are almost nil,and specially when you were expecting your periods nearly at the same time so.
Brownish discharge is due to the ipill which is a hormonal
pill and it indicates that the pill has worked so it is common after that pill.
Regarding precautions just see to it that you don't have unprotected intercourse
and if ejaculate falls on any of your body parts clean it immediately. Avoid using ipill S it has its own side effects.