There are several possible reasons for the rapid gain and loss of flexibility you've noticed. First, you may be experiencing what scientists refer to as creep. Various elastic tissues in the body have a certain amount of laxity or creep. This applies to ligaments, discs, joint capsules, and connective tissue covering the muscles.

Staying in one position for long periods of time can increase or decrease creep in the lumbar spine. Creep describes the change in the viscoelastic structure when under constant load. The tissue may contract or tighten causing micro damage in the collagen fibers.

Second, the discs are under a lot of stress and load when the spine is flexed forward. During the upright posture the discs take on fluid to stay as full and supportive as possible. Bending forward squeezes fluid out of the discs. This may help explain why long periods of forward bending can get your fingers closer to the floor but also leads to stiffness and injury.

Finally, muscles have elastic properties, too. During prolonged stretch the fibers of the muscles may relax and get longer giving you greater flexibility. Once you return to an upright position the muscle fibers return to their normal resting length.