Sir I Have Unprotected Sex Last Monday Morning And Now

sir I have unprotected sex last Monday morning and now I am worried of my gf pregnancy is it possible to her geeting pregnant. and her last period ends on last Sunday and we had sex on monday what should we do for surence we had home pregnancy test after 2 days of sex it comes negative what medicine should she eats and the medicine will harm her also she is about to be 18 and we are not sure that sperm enters in her vagina so in this case what should we do whether to give pills or not if we have to give then which medicine and what doases and what are there side effects we belongs to India so plz sugesst us Indian market medicine and one more questions is that can we Waite till a month to pass if her period doesn't comes then we may give her medicine