I pulled a muscle in my back and now when I lay on my back my left leg goes numb?

Sciatica. Sounds like you have sciatica, which is a syndrome of lower back pain with leg numbness, weakness or pain due to compression or irritation of sciatic nerve. It is commonly caused by disc bulging. Your sciatic nerve could be pinched due to tight muscles in your buttock (pyriformis muscle) or pelvic trauma. Would try anti-inflammatory or pain meds and heat/ice. Physical therapy is helpful for this.