Don't know what is going on with my GF.

Hello there! Me and my Girlfriend did an outcourse on 25th Dec and her period date was 27nov last month her period are not there I consulted a Dr. Online they said that 2 days before periods and Protection, although I penetrated and her hymen is stretched completely but didn't ejaculated inside her we just rubbed to get that body to body Feel and then I checked that no Pre or post ejaculatory fluid is leaked or not everything was fine she still not getting periods we had checked Urine Pregnancy HCG test it said negative what is it going on! Is hymen stretching affect periods? As body go through some changes I m also a medical student but still not getting anything as I am An aspirant have basic knowledge any all the things please help me Sir/Madam! Regards Aniket Anand