What can cause pain in my buttox with a large knot and pain that goes down to my foot?

Sciatica: Pain that radiates down the legs can mean a nerve that is getting pinched or perhaps a herniated disc. I would see your pcp for further evaluation.You may need some xrays and referal to p.T.

Large knot?: The "large knot" part of your question is a little concerning for something like an abscess or a furuncle. "pain that goes to my foot" sounds like sciatica. I'm having a hard time coming up with one diagnosis that links both of these symptoms. Sounds like a visit to your pcp is just what the doctor ordered.