I am 18 male, having pain in the pelvic bone and experiencing cramps as well. Have showed doctors and all x-rays and urine test clean. Tried miralax but no effect and taking advil. Bowel movements are still regular. What's the reason?

Pain & cramps. Please keep looking and get a second opinion, perhaps from a GI doctor. How is your diet and exercise level? Consider food sensitivity lab testing like the one at Great Plains Lab in Kansas. Peace and good health.