For a week i've had moderate-severe lower back, hip & upper thigh pain with no known cause or injury. it feels muscular, but not 100% sure - thoughts?

Lumbar disc injury: We need a bit more info: do you have any numbness or tingling going down your hip/thigh? Does the pain get better with rest and/or worse with activity? Most likely though, this is due to some injury to one of your lumbar discs whereby nerves get pinch and inflamed. This could be treated with anti-inflammatory medications, heat/ice, gentle stretching, and also pain meds. See doc if not better.