What Causes Pain On Left Side Post Pericardial Window Surgery?

Two weeks after surgery under GA for prticsrdial window. By evening such pain on whole left side I can hardly move Have lost lotion weight but appetite is good again. I am 5ft 7in and weigh 116 lbs normally weigh 126 lbs am taking oxyodone for pain and oramorph 2.5mg at night. Still in pain. Like knives pushed into side and turned I'm a lady 71 years and normally healthy. Underlying health issue of BC. But that's not the problem. The problem is the pain from the operation peticardial window .