Given the information you provided, it seems likely that you might be experiencing “bleeding in the middle of your cycle”. Although there is no info about the usual length of your whole menstrual cycle (that is: from the day of the onset of one period until the day before beginning of the next menstrual bleeding), but since you said your cycle is regular and given the date of your LMP-last menstrual period, it is presumed that the day of the explained sexual “relation” (not an intercourse) might have been exactly the day of your ovulation.


Various women have what is called a “stronger ovulation”, accompanied by a greater hormonal disbalance (intensive dropping of blood estrogens while raising of progesterone level is not that fast, just before ovulating), which, all in all, might result in brownish spotting during these few fertile days in “the middle” of a regular cycle. If this is only light spotting, it is considered pretty normal.

Still, you might want to consult with your gynecologist to have your hormonal blood levels checked as well as do an ultrasound, if necessary. You also might want to do a pregnancy test if your period is 5-7 days late, although the very acidic vaginal secretions leave extremely slim (almost theoretic) chances to get pregnant if ejaculation was outside the vaginal opening, even though you could not find “where the semen went”.