Bf only inserted tip of his head, no sex.late period, brown discharge, tiny light brown blob came out. it's a week since the incident.why ?

Difficult to say: Even the tip of the penis can cause a woman to get pregnant. If you've missed your period, which it sounds like you have, then you want to check a pregnancy test. The spotting is difficult to say... Maybe due to hormonal changes. Maybe it was a precursor bleed and you'll have a full period any day now. Check a test now, and make sure not pregnant.

Irregular bleeding: Irregular periods are one of the most common problems obgyns treat. If this is one time problem it usually resolves on its own. If this is a recurring problem you should be evaluated. Infections ; hormonal changes can cause spotting. If there is a concern for pregnancy then do a home pregnancy test . There are many options available to help regulate the menstrual cycle. Schedule a visit.