What is it

If you know you were under your calorie goal, it can't be fat - water weight can vary for any number of reasons. You may have consumed more sodium, a higher percentage of carbs, if you start an exercise program, hormonal reasons (especially for women) or even if you spend the day in the sun!

It is absolutely normal. Don't let it bother you. Keep tracking and be patient.

Have you taken up exercise, a new exercise, or did you overdo an exercise?

Soreness can add temporary water weight while your body makes muscle repair. It can be substantial, too- several pounds worth.