Impression on pet CT scan is listed as: follicle on right ovary with associated hypermetabolic activity. What does this mean? Should I be concerned?

Not sure. What really matters is why they did the imaging in the first place. I am going to guess that since you have a history of breast cancer, they were looking for possible distant metastases of this disease. I have personally seen breast cancer spread to the ovaries. If this is their concern, it should be further evaluated.
Depends on cycle. It depends on whether you were ovulating during the PET scan. If you recently had your period then it is just a normal finding on the scan which shows that your ovaries were releasing an egg from the ovary. However if u have cancer and it has metastasized (spread to other parts of your body ) then it may be a more concerning finding. The doctor reading the scan should know when your LMP was. .