Have you done a pregnancy
test to rule out pregnancy as you are late for periods. Get one done immediately. You could also be having a problem of PCOS
if cycle are irregular so get an USG and Hormonal
assays done.so the delayed period could be because of PCOS. The brown discharge could either be blood that failed to pass out during your previous cycle or due to minor trauma to your cervix
or vagina during intercourse and found its way out late.