Had regular period starting in the 18th then had sex on the 27th then on the 31st had dark red/ black brown color flow for only two days. what was tha?

Random old blood: My guess is that for whatever reason, your body did not expell all the blood/lining of your uterus this month and then for another random reason (maybe/maybe not due to intercourse) the leftover blood,which would naturally be darker and/or black/brown because it was in there longer than usual,was finally expelled over those 2 days.

It could be: left over old blood in the vagina and the vagina is cleaning itself out.