What Causes Tailbone Soreness While Sitting?

I have had a tailbone soreness when I sit for long periods of time or when I push on the bottom left side of my tailbone. It goes away upon standing. There was no injury associated with the beginning of the pain and it s been going on for 6 months now. I ve had an xray and it shows nothing. I am in good health and normal weight. What could this be?