It is impossible to gain a pound of fat over night. You are probably just retaining some water.

As as reminder......16 oz equals a pound. Normal water intake is 64 oz or 4 pounds. Your daily weight will probably flux 1 to 6 pounds a day depending on what and how much you eat.

Take a deep breath and relax. If you are going to weight every single day, may I suggest you write it down then average it out 7 days at a time