What Causes Pain In Site Of Lithotripsy Procedure?

Two weeks after lithotripsy I am having problems at night sleeping. I go to bed feeling comfortable but during the night I start experiencing pain on the left side where I had the procedure. I have to get up from the bed and just sit up trying to sleep hoping the pain will go away. Sometimes this helps but today the pain has not subsided since late last night. Am reluctant to take the pain medication since it caused me to have a severe impaction. What causes the pain? Why am I experiencing going to bed pretty comfortable to a few hours later being in pain?