What Lead To Only One Day Of Menses?

Hi doctor...my gf is just had a period of only one day...i am quite scared as we had messed around but havent had'nt actual sex.....although i doubt that my semen has touched on the outer of her vagina.....but still i think she is not pregnant......Can you please advise as to what could be the reason for one day period...because i think semen touching vagina does not leads to pregnancy......