The temporary weight gain explained:

When someone starts a new exercise program, they often experience muscle soreness. The more intense and "unfamiliar" the program, the more intense the muscle soreness. This soreness is most prevalent 24 to 48 hours after each workout.

In the first few weeks of a new program, soreness is the body trying to "protect and defend" the effected or targeted tissue. Exercise physiologists refer to this as delayed-onset muscle soreness, or DOMS.

This type of soreness is thought to be caused by tissue breakdown or microscopic tears in muscle tissue. When this happens the body protects the tissue. The muscle becomes inflamed and slightly swollen with fluid retention. This temporary retention of fluid can result in a 3- to 4-pound weight gain within a few weeks of a new program.

Keep in mind that muscle soreness is not necessarily a reflection of how hard you worked. In fact, some people feel no signs of muscle soreness, yet will experience the muscle protection mechanisms of water retention and slight swelling.