spotting after morning after pill

I would like to know if anyone has had brown and light red spotting after the morning after pill for at least a week and still become pregnant? I have taken it before many years ago and never had bleeding and wasnt pregnant but this time I've had brown spotting for over a week. I was taking St Johns Wort at the time or using the MAP but it was only a low dose and the chemist said it should be fine. Thank you for any replies