My husband had the same thing at the end if January. Coughing so hard non stop an could not breath. Took him to th E R. They did a flu test step and one other test all came back negative.Oxygen levels were low so they did breathing treatments. Xray showed small spot on lower part of lung which the Dr.said could be the beginning of pneumonia. Went home with zpak albuterol and something for the cough. It took about three weeks for him to get better.The dr said it was viral whatever it was.Everyone in our family caught it also.
There are many strains of flu floating around, and the tests do not catch every single one. Pneumonia is a common complication of influenza.