hello i suffer severe back pain through prolaped disc and you have a sciatic nerve at tailbone of spine,,this can cause alot of problems if irritated..normal x-rays do not detect spinal problems too well so yes,,mri scan is best..any back complaint is worth investigation and maybe all you shall need is gentle physiotherapy to sort it...only a scan will truly detect your painful problem and next step to take action for it..
I feel for you as im now three yrs chronic sufferer.
good luck