I've never heard of N2 affecting chlorine readings, and it sounds like you were testing shortly after adding the N2, so it probably had barely begun to release into the system. Assuming you let the pumps run at least 15 minutes on high to allow the chlorine to dissolve and mix thoroughly before testing , I can think of two possibilities. The first is that 2 tbls of chlorine is a quite a bit (8-15 ppm, depending on the size of your tub), so it is possible that the high level of chlorine was bleaching out your test strips. The second is that there was bacteria in the water left in the plumbing when you drained the the tub, and that two days without sanitizer was enough time to allow an infection to grow to the point that it "ate up" all the chlorine you added.
If there is still no reading a day later (or however it would normally take that much chlorine to dissipate in your tub), try adding a 3 ppm dose and see if you get results. If you do, I'd blame yesterday's lack of reading on excess chlorine. If not, either verify the results/lack thereof with another testing method (preferably a dealer with electronic testing), or assume an infection and keep adding chlorine until you can maintain a residual.
In the future, I'd get some chlorine in the tub sooner than two days after a change. I personally like to do a chlorine shock the day before I drain it to make sure that I am starting with a clean tub, and then add a normal dose within a day of filling the tub.
