What could be causing my upper back pain?

I have had a sharp pain in my upper back for the past two months now. The pain comes and goes but is not always caused by anything. The pain is strengthened whenever I bend over, get up from laying down, hiccup, breath heavily, etc. The sharp pain has been present in my back constantly for the past two or three days now and tends to do this often. Occasionally, it will not be so obvious for about a day.