had a contraceptive pill, any reason to worry?

While it is possible to get pregnant during day 4, it is very very very unlikely, unless you have cycles shorter than 23 days.

An orgasm can force your uterus to start menstruating again when the contractions ripple through the uterus wall. The brown blood is normally just a sign that it is older blood. Some women experience this at the end of their periods when flow is light and the menstrual flow take a while to exit the cervix. The pill that you took can also cause some withdrawal bleeding.