It could be fluid retention from being in bed, and changing your diet, along with your period.
I don't see how it could be the flu, but if it lingers, and you don't lose it after getting better and moving around, etc., see your doctor.
I am going to guess that is a fluke. Popsicles and crackers aren't going to cause you to gain weight. The constipation may, the period always does, etc. I'm wondering though why you are checking your weight when you have the flu. Do you obsess about weight a bit? I have had periods of my life in which I did to and my equally obsessed girlfriends would joke we need a good case of stomach flu to get us back on track. Bad joke, right? But I'm sure that was a bit unnerving when you think you probably lost a bit from being sick. I always remember that my body doesn't always show on the scale. Like when you are going for a weight loss and you've increased your exercise. The scale isn't changing but your pants fit better. The two (scale and body) aren't matching up. And then all of a sudden they do. So, don't take a few scale readings in a bad way. (also, check to see if your scale was off). Let us know if you are back to normal!