After doing cardio, scale sometimes goes up a pound. Do muscles swell? Initial gain is discouraging, but usually see a loss few days later.

Fluid? Do you drink (water or other replacement) when you exercise (as you should!). Don't weigh yourself during the day: there are too many variables. Always weigh yourself first thing in the morning, after emptying the bladder fully, no clothes, same scale, same place, same time. There will still be variability depending on diet& bowel movement factors - so once a week is as often as you should check.
Probably water. Weight fluctuates day to day and different times of day; probably more related to fluid intake, or mild volume depletion. In general though, as muscles develop, weight can go up some. Probably better to look at your average weight about once a week. Keep up good work, don't let the scale frustrate you!