I rarely have periods, maybe once a year but I recently had been having unprotected sex where he ejaculated in me. I now have been spotting on & off with a light pink to brownish color for over 3 weeks now! What does this mean?

May mean nothing, spotting for three weeks in and of itself is not a sign of anything.

It is too early to do pregnancy test, so really need to wait for another 3 weeks and try a test.