2 days ago, coughed and felt massive pain in my lower back which made me fall, after hour on floor manged to crawl to the sofa now i can stand but left leg wont straighten so cant walk, back pain still bad but getting better, also side of bum hurts?

This history suggests you may have a herniated disc in your back with compression of the nerve on the left, or at least a lot of inflammation around disc causing irritation to the nerve. Best to have Dr eval, possibly some steroids short term, & physical therapy. Meanwhile Ice to the area every few hours & anti inflammatory cream & meds can help, & gentle movement (NOT prolonged bedrest )