Just as nerve root problems in the lumbar spine can result in sciatica (pain in the thigh, lower leg and foot), so problems with the nerves emerging from the lower spine (sacrum) —e.g. pinched or inflamed at the point where they exit the spinal column— can cause pain in the pelvis.

One way of testing if your problem is spine-related is to get an ice-pack (gel-filled is preferable, but solid plastic will do), which is about as large as the area indicated in the above illustration.
Cold (ice) therapy is used to combat numerous inflammatory conditions, and is known to increase the levels of anti-inflammatory proteins (cytokines) and decrease that of proinflammatory cytokines. I surmise that just as heat can change the nature of a protein (for example, cooking an egg), so cold can too.
Product links:
https://www.amazon.com/dp/B0027CU1EY
https://www.amazon.com/dp/B0000DH4M3
Tips
- Place the ice pack on the lower spine and include the area of the upper buttocks so that top half of the "crack" is covered ;
- Make sure there is a thin layer of material, such as a single layer of cotton T-shirt, between your skin and the ice pack, to prevent frostbite. A thin bed-sheet is too thin, and most towels are too thick and insulating (making the ice pack ineffective);
- Lie on your back on a bed with knees raised;
- Place pack under your spine as indicated and stay there for 10-12 minutes. TIME YOURSELF!
- At about the 5th minute, the discomfort caused by the ice pack will cease as the area numbs;
- Repeat treatment every few hours when the skin in the area is warm to the touch again. Do NOT re-treat when the skin is still chilled in any way.
- It is very important NOT TO GO OVER THE INDICATED TIMES! Exposures of 20 minutes and more can result in FROSTBITE (Google "ice packs frostbite" if you don't believe me).