If your brother has colon cancer and with the symptoms you are saying I would talk to your doctor to schedule a colonscopy, that way you can rule out any cancer, polyps etc. It is possible that you could have Rectovaginal endo as it causes vaginal and rectal pain. Have you ever had an ultrasound?
look up piriformis syndrome and levator spasms...pelvic floor dysfunction ... it could be taht...it's hard to say but it sounds like the muscles are in spasm which sex definitely causes for me...and has for over 20 years...but if in doubt definitely get in to see your Dr. Maybe see if your dr. could get you in sooner if they have a cancellation...you might try putting some heat on the backside and elevating your legs so the knees are higher then the hips as this forces the pelvic floor muscles to relax...see if that helps...but if you are to ridden with worry...put your mind at ease and do what you need to do...let us know!