Everything is in a state of flux, or change, and war and strife between opposites is the eternal condition of the universe.
The human body too, submits to its own unique array of dynamic tensions. Between all parts there is constant adjustment, coaction and strain occurring, to keep you as close to a state of equilibrium as can be achieved against an always changing stimulus, or environment. Perspiration, elevations in breathing and heart rate, losses and gains in body mass, are but some of these wonderful harmony chasing mechanisms at play. Stability is the goal, and an ever-moving one at that. The pound you gained or lost since yesterday may not mean all that much, so bring your thoughts around to the idea that there is a chance your body is according perfectly to its design. You will never twice inhabit the same body. Have this faith.