What Could Possibly Cause Delay In Periods?

i m 27 m before 57 days i did sex with my girl friend and after 50 hours she take p not pill and after 7 days shes periods come but shes blooding only 2 days come & actually shes periods date is 13 to 16 and after 57 days i did sonography but reports are show normal but still 49 days shes periods not came,it is possible shes is pregnant.