Sorry you weren't feeling good. Are you sure you had actual influenza and not just what people call "the flu"? There is a BIG difference. It sounds like maybe you were just dehydrated and that's why you needed the IV. Coughing and legs hurting aren't typical of influenza. If it was the true influenza virus you would be out a lot longer than just 2 days and would be very serious this late in pregnancy.
Also, just like any vaccination, getting the influenza vaccine doesn't make you immune to every strand of it. They change the vaccine yearly to combat the strains that they think will be the most prevalent but that doesn't mean it hits every strain out there.
They did the nose swab thing and it came back positive for the flu. They only released me today because I'm able to eat and taking fluids good. Still feel crappy. I think they kept me last night because I had a fever and was dehydrated plus they wanted to monitor him and make sure he was doing fine which he was/is!