Am I pregnant? What could this be?

Hannah

Okay so my period was due for the 18th of August, on the 19th I had a slight brownish/reddish drop of blood but only a little. I haven't had anything since that. During this month when I was on my ovulation week I was sexually active quite a bit using withdrawal method, I've also read stuff about implantation bleeding. What would you say this is, is it worth taking a pregnancy tests or is this normal? How long should I wait until I take a pregnancy test?