Fluid retention can be caused by adrenal malfunction, deficiencies of potassium, vitamin
B complex, or
vitamins B1, B3, or B6, food allergy, hypothyroidism, anemia, constipation, and lack of exercise. It could also be due to carpel tunnel syndrome. While lying down the circulation slows down and hence fluid retention is seen in hands and at the tail bone.
Hope this helps. It is difficult to comment beyond this at this stage. Please let me know if there is any thing else and do keep me posted.