What Causes Pain In The Tailbone While Sitting?

Hello! thank you for your time. I am wondering about pain I ve been having, for a few months now. The pain is located in my tail bone area. I notice it when I am sitting, especially when sitting back in my chair, rather than forward in my chair. I have seen my doctor, who examined the outside of the area, concluded it was a bruised tail bone, and encouraged me to get a better computer chair and to notice if using the stationary bike was causing the soreness. 6 mo later, the pain is still here. I avoided the stationary bike and have been using a coccyx cushion for the past 2 mo, which helps with discomfort, but doesn t seem (yet) to be alleviating the cause of the pain. I recently went back to my MD and she ordered an X-ray of the area, which showed nothing remarkable. At that point, my doctor suggested physical therapy of the pelvis. When I expressed concerned that this pain might be a symptom of something more serious than a bruised tailbone, perhaps a tumor somewhere in the area, needing more diagnostic inquiry, she prescribed an MRI of the pelvis, contrast and non contrast. I wonder if there are other diagnostic measures that could be taken first, as I worry about the risks of the contrast medication. (I may have had a contrast MRI once in the past, following surgery for removal of a section of my colon due to diverticulitis.) Also, if I do have a tumor of the bowel, rectum, or anus will a pelvic MRI reveal that? Thank you so much, Connie