The swelling (bump) you described does not appear to be a malignant tumor. Tumors either malignant or benign do not disappear and reappear or change in size within hours. Your doctor must have already explored every possibility of concern and ruled them out.
Pilonidal cysts are common in that location near tail bone between buttocks. But, they usually are very painful and have an associated sinus tract which drains the cyst to skin surface. Absence of pain and draining sinus rules out pilonidal cyst. A milder form of spina bifida (a condition where is congenital fault in closing of vertebra around spinal cord) called spina bifida occulta can present as a solitary fat lump in the lower back but it is present usually since child hood and also a little above the position of your swelling. Anyway, ultrasound will rule out spina bifida occulta. Most probably the swelling is nothing but some abnormal collection of lymph or blood due to defect in lymph channel or some blood vessel. The decreasing and increasing of size may be due to pressure and position of your body which affects the fluid collection. Or, it may be a small lipoma and the change of size and shape may be due to pressure of surrounding muscles and tissues. In either case there is nothing to worry about. Follow your doctor's advice and wait to see if it shows any growth.
Hope this helps. Thank you.