Bump or pouch of fluid on lower back.

Patient: I lost control of my bike on a bumpy hill last summer (about 6 months ago now) and landed on my back. I already had back issues, so I went to the chiropractor to get adjusted when the bruising and scrapes healed. However, one thing still hasn’t gone away, there is a ball of what I’m guessing is fluid at the very top of my bottom ( on the sacrum). It is sometimes very uncomfortable (I can’t lay on my back , or lean against things). Do you have any idea what it could be? The “ball” (pouch, bump) is roily two and a half inches wide and about two inches from top to bottom. I live in a country where I don’t trust my local medical system, and to get someone here with decent doctors is a long journey, so I tout I’d try to find out more before either making the trip or finding someone close by. Thanks,