Either could be the right answer. Infections after treatment are common, your immune system is beat to death by all of the treatments and every opportunistic bug, fungus, or virus wants a piece of you...and you have little left to fight them off. For me it was one thing after another for the better part of 6 months. I was always on antibiotics for something, then on antifungals for the blooms of Candida that the antibiotics let happen. I got every virus that went around, in spades compared to regular people that got it.
CT scans while they show changes in tissue mass, cannot diagnose cancer; only a biopsy can give you a definitive answer. CT scans will also show changes resulting from the treatments, including in my case nodes which were positive in the CT but after surgical removal proved to be cancer free...zillions of rads of radiation will kill the cancer, but to a CT scan, it can't tell if the cancer is alive or dead, just that the node is not normal. The tissues after treatment are certainly not normal, there is lots of swelling and other changes that the CT can not interpret.
Try not to get too upset about all this until there is a finite answer at hand. From my own experience and that of many that I talk to, that first year after treatment is full of surprises, most of them not cancer. This doesn