I'm scared I might have a deeper internal issue with my organs then the doctors have looked for.?

If the pain is starting in your tailbone, is it possibly nerve damage or spinal related problem, that maybe the cyst has nothing to do with your pain at all. Drink more water if you are concerned about dehydration, and with the ongoing weight loss, it's common to have irregular periods because of the influx of hormones. I really wish you the best.