one tiny little dot of blood what does that mean?

Hi, and thanks for your medical question on eHealth forum!

Since you had intercourse around 12th -13th day of your period, you could have conceived. The spotting could be implantation bleeding. Wait until your next due date for periods and if you miss them take a pregnancy test and see. It could also be ovulation bleed, if your ovulation comes around 18th day or so. Hope this helps. Take care!