I Had A Really Really Serious Fall Of My Horse

I had a really really serious fall of my horse last Saturday. I did go to the hospital and they didn't really tell me what I had done but they let me go home with severe severe pain in my lower back. I have a really bad weakness in my bAck anyway which I went to the doctors about a few weeks before my fall. he has sent me off to a spinal unit and hoping I will be sent off for an MRI. I have a huge lump on my lower back now which is extremely painful and feels like an inflated muscle? What is this?