In fact, I am researching on this question as well, but I may have a possible cause. About two months ago in a high school football game, I jumped up and landed completely flat on my back on the artificial playing surface. Although I had a back pad, I experienced this "fluid filled swelling," and still have days with alot of swelling. One possible cause of this could be from a rupture of the protective coat that surrounds the muscles that surround your spine. If anyone has any more detailed explanations or a different one I would be happy to read your posts.
Back pain lower right side causes are: Lower back and kidney disorders.
The driving force that pulls interstitial fluid back into the capillaries is primarily osmotic pressure created by proteins in the blood, such as albumin. This osmotic pressure causes water to move from areas of lower solute concentration (interstitial fluid) to areas of higher solute concentration (capillaries), helping to maintain fluid balance in the body.
You could have a slipt disc in your lower back, they are very painful.
sciatica
There can be various reasons associated with this. Injuries, infections, fractured bone or tumor are some of them.
One can find information about the causes of lower back pain from the following sites; Chronic pain site, WebMD, Mayo clinic, Bupa, Health guide, and NHS.
Possible causes of lower back pain that can result in the inability to lift the leg include herniated discs, sciatica, spinal stenosis, and muscle strains.
Possible causes of lower back pain when lifting your leg while sitting include muscle strain, herniated disc, sciatica, or spinal stenosis.
The principle force that causes movement of fluid from tissues into capillaries is oncotic pressure. This pressure is generated by the presence of proteins in the blood that draw fluid back into the capillaries by osmosis.
LABOR
Common causes of lower back pain when walking uphill include muscle strain or fatigue, poor posture, improper footwear, and underlying conditions such as herniated discs or spinal stenosis.
Legs can lose fluid due to several factors, including gravity, which causes fluid to pool in the lower extremities, and conditions like venous insufficiency that impair blood flow. Additionally, prolonged inactivity or standing can lead to fluid accumulation. Other causes include certain medical conditions such as heart failure, kidney issues, or liver disease, which can disrupt fluid balance in the body. Dehydration and excessive sweating can also contribute to fluid loss in the legs.