What Causes Pain In Elbow?

I have unexplained pain in my elbow. It started out feeling bruised but no marks. Now it has constant pain in it but hurts really bad to touch it. Still no discoloring. I did not hurt it. However, I have been sitting and sleeping in a recliner for the last week because I just had back surgery. So my elbow has been bent and or propped on alot this week. What is wrong with it? And how do I fix it?