PostOp care

We have a patient who was in the hospital for upper endoscopy, suction rectal biopsy, NG placement. Patient came to our office with symtptoms of fever, vomiting, diarrhea on multiple occasions to be seen. We are billing an office visit with the admission and discharge dates included on our claims. The insurance is denying our claim's for global. The above dx codes are the same symptoms the patient had when the they originally were admitted to the hospital. After reviewing our documentation there are no other dx codes that weren't related to the reason the the patient to be admitted. We resent our claims back with a 55 modifier on the office visit and the insurance again is denying our claims for "postoperative dates begin the day following surgery." After doing some research I read that we have to bill the same procedures codes and diagnosis codes that the hospital used, even though services were rendered by the patients primary care doctor. I really need some help..this patient has been in and out of hospital since Feb this year and all our claims are being denied by the insurance for global.