Hi ! I have gone through your question and I understand your concern
I would say that Harvoni drug have not shown any side effect in your back .
I would ask you a few question first : How much does the pain last? ( as I understand only few minutes , after you get up and move the pain is gone , right ?) , Does it last more than one 1 ? , Does the
back pain wake you up during the night ? Do you have any family members that have suffered from any reumatology dissease like Ankylosing
Spondylosis or any autoimune dissease?Do you have eny other symptoms like joint pain , swallen or stiffnes? .
If the answers are no , I would say that you dont have any sign that can show me for any inflammatory joint dissease.
I would say that the morning stiffnes is caused by the muscles tightness during the night sleep . Your low back muscles become tight during the night and put pressure on the joint resulting in pain . This condition may happend from the posture , you mattres , cause of the type of work and exsercise during the day before . I would recommend to change the posture ( while you sleep have a streight position) the mattres to feet you body structure. Do some exsercise The standing too short exsercises .
The thing in concern about is
hepatitis C arthritis ( And to be exsclude that you should do the virus
examination in blood HCV-
RNA . You should Do that also while you finish Harvoni drug.
Hope these help
If you have any other question please feel free to ask me .