I have ight discolered (brownish)blood in my underwear 38yr oldmale. could it b from sex while my gf was on her period? even after discharge?

Good guess: I suspect that some menstrual vaginal secretions from your girl friend were deposited and dried on your underwear. By "even after discharge", I suspect you mean even after you had ejaculated. Yes, ejaculation would not necessarily have eliminated all secretions on the outside of the penile shaft. This all sounds normal and nothing to worry about. Good wishes:)