Hi there!
Well, without a detailed evaluation it would be difficult to comment specifically on the situation. Possible causes of the symptoms could include neural pressure at the spinal level or local causes at the hip/ knee. An orthopedic evaluation and probably an MRI may be required to assess the situation to arrive at the diagnosis, so that a management plan can be drafted. I would suggest contacting the social services department in your area and see if they would be in a situation to provide medical aid in the situation. Meanwhile I would suggest rest. Warm compresses and tylenol/ OTC NSAIDS may be helpful for pain relief.
Hope this is helpful.
Take care!
In the meantime, you can try a memory foam cushion with the coccyx area cut out to help you eliminate the pressure in the coccyx while sitting.