Help please periods not coming

Patient: My gf took 2 periods postponing tablet last month 3 and 4th jan ..and periods start from 6th… now its 13th feb still her periods not started…we also had unprotected sex on 11 and 19th jan…. she is feeling periods pain but periods not starting … 13th feb she take HOME PREGNANCY TEST and its NEGATIVE(not pregnent showing)..what is the problem… ? can postponing tablets play with next month periods date.any serious problem plz HELP?

Symptoms: Feeling pain like before periods