What Causes Pain In Tailbone And Pelvic Area After An Injury?

i was running and doing wind sprints and as i touched the ground and went to turn around, as i started running back, i felt a pop on my left tailbone area where my tailbone and my pelvic meet. my dr.'s took an x-ray and didn't see anything broken. they put me on alieve and muscle relaxer. i am still having pain in my tailbone and pelvic area, i want to know what i could have and should i get an mri, i'm in the military and it is easy for me to request one. i do the exercises that they say should help but some times it makes it worse