Dieting and protein powder??

I’m not very good with reading nutritional facts so I really only watch my calorie intake. I go to the gym everyday and just do some like workouts like walking on the treadmill and the stair stepper. The past two days I have been working out my leg muscles very lightly. I was losing weight every single day, but today when I checked I had gained back two pounds. I didn’t overeat yesterday at all and definitely not enough to gain back two pounds, so could that be muscle gains? Also I wanted to know if I added a protein shake to my diet before I go to the gym or like for a meal replacement for breakfast would that make me gain weight. I’m not sure what protein does to your body, but I know it’s good for the gym I’m guessing.