I agree, it could have been the very end of her period. Not all women ovulate in the middle of their cycle. It's best to use protection every single time with every single sex act if you do not want un unexpected pregnancy.
My other question is since she bled, was she still in her period, thus making her unable to be impregnated, or does the leftover blood have nothing to do with whether her period was over or not?
She could have a variety of STD's, or a simple yeast infection, and it is possible it was an end part of her normal menstrual cycle, a visit to the doctor is wise, but so long as you know that neither of you have an STD it's unlikely she contracted one, also leftover blood from her normal period could have become dislodged during sex, It is not crucial, but I recommend she see a doctor.