When you exercise regularly, your body stores more glycogen to fuel that exercise. Stored in water, glycogen has to bind with water as part of the process to fuel the muscle. That water adds a small amount of weight, too.

Why am I gaining weight when dieting and exercising?

Body weight tends to fluctuate by a few pounds. It depends on the foods you are eating, and hormones can also have a major effect on how much water your body retains (especially in women). Also, it is possible to gain muscle at the same time as you lose fat.

Is it normal to gain weight before losing?

But before you launch the scale (and your resolve) out the window, know that it's totally normal. In fact, it's not always a bad thing (and it's often resolvable when it is). Sometimes, putting on a few kilos is part of the process of improving your overall body composition.