What Causes Tightness, Stiffness And Numbness In Back?

Hey, I just had a few questions regarding a strained lower back. I was hunched over for an hour and a half busy working on something. Later that day I noticed a weird feeling near or on my spinal cord. It felt short of numb or pulled. Anyway I went to the doctor a few days ago after coming from work. I probably shouldn't of gone to work. Anyway she came to the conclusion that its a strained muscle(s) in my lower back. Now my back feels tight and stiff (pulled) but I am also getting a weird numb like sensation, is this normal? It only is in my lower back and partly into my butt.