What Causes Swelling In Face Along With Dizziness Post Tumour Removal Surgery?

My dad is 64 and has had small cell lung cancer that he was treated for, and he now had a tumor removed from his brain that the doctors said was a result of the lung cancer. I was under the impression that this was going to be a simple operation and he was going to recover fine. I brought him home 2 days ago from the hospital when he was doing just fine, and now 2 days later he is not doing so good. His face is swollen, he wont eat, he is dizzy all the time, and today, he slept all day. I am worried. What should I be looking out for?