Hi!
enagel is correct that intercourse could cause bleeding. If you were my patient, I would advise you that sometimes a small separation of the placenta (sometimes referred to as a marginal sinus separation can cause bleeding too. At term this is not necessarily terribly worrisome. I would watch you carefully in labor and have a low threshold for a cesarean if you had any significant bleeding. I'm sure your doc will do the same.
You should also report any further bleeding, as you have done in the past.
Good luck!
Dr B
Did you have intercourse the week before it happened? That could do it. Or maybe you just have a sensitive uterine wall and the weight of the baby at this point is causing some stretching and in turn some bleeding? I'm not a doc so not sure but if the hospital sent you home I am sure its okay. If you're worried maybe invest in an inexpensive Doppler at babies r us so you can check the baby heartbeat every day at home til it comes?