as u states u are taking medicine as u need,which is more than ,prescribes at yours age,overdosing a medicine can cause this side effects- and diplopia
is also often reported,, stop the medicin & donot use any pain killer for few days,instead u may apply local muscle relaxant
creams & oils.do hot fomentation,use lumbar coarset belt while driving,mild back exercises wiil be good to prevent stiffness
of back . u can go for short wave diathermy which will give u good relief.