Confused??

My fiance still hasn't got her period and she was supposed to get it on December 7 and it is now Dec 20th don't know what happening been 13 days late and she did NI with our sperm donor on November 13th and she took a test on December 13th and it came back negative like WTF?? But she having preggo symptom's..though I don't get it... Maybe to early or what? Helppp??