What Causes Pain In Buttock?

I have a paid in the bottom on the right side. By the end of the day, I have a fairly sedentary job, I am unable to continue sitting at my desk. I feel it when I walk - I have a dog and enjoy the outdoors. It has been getting worse for the last couple of years - more so in winter when I wear wellies and struggle to get them off. If this is piriformis syndrome, as I think it might be, should I rest it or continue to exercise?