My understanding is the radiation should have destroyed the entire tumor, at least that is the goal. That said, the body can and often does take months to eliminate the dead tumor cells. Sometimes they are never completely eliminated and a scar forms around that which remains. The scar can and often does show up on a PET scan as well as MRI which is often alarming to the patient but it should not be. An additional scan will show any progression or not, and we hope for "not".
The body's recovery from radiation therapy is rather slow, similar to that of a severe burn but on the inside. Good cells, nerves, and tissues are damaged as well. The body must deal with those systems and the entire rebuild can take years actually. There are many good tales on this site of slow recovery and what many patients have endured, myself included. In the end the sun often shines again.
Are you having unusual issues or something you are not sure of?