I have severe pain in my mid to lower back, it feels like a bruise and hurts constant, also pains down my left thigh what could this be?

Need more evaluation: Commonest reason for pain radiating down legs is a 'pinched nerve' because of a displaced disc. This can happen in young people like you. You may need Xray, MRI or nerve conduction studies followed by either medication,physical therapy or nerve blocks. See a board certified pain physician or an orthopedic doctor

Sciatica: This sounds like sciatica. Pain in low back that radiates down your leg is very suggestive of sciatica. Use ice Advil (ibuprofen) and stretching. If not better in 24-48 hours see your doctor.