What Causes Sharp Pain At My Back?

I was helping A friend fix a part to there car . I was standing on a crow bar 2 give this car part some lavage, I didn't know that they was going to strike the part! When they did my body tense up and sharp pain shot up my back. It has been 4 days since it happened, my neck and shoulder is stiff and the rest of my body have a tingly feeling. Is it normal for a person body to tense up when frighten?