AF 4 months late BFN!

Carole β€’ πŸŒΈπŸ’•
I'm posting this for a friend who is really confused. She's been to the doctor multiple times because her period is 4 months late, but every single one of her tests have come back negative. Her last period was in November (not sure of the exact date). 
Have any of your ladies had this problem? Is it possible to be pregnant with all of the tests coming back negative? Any advice to her would be helpful! Even her doctors are dumbfounded. Thanks!