It is impossible of course to tell what this is without a biopsy, which could be accomplished with something as simple as a fine needle aspiration quickly. Given that you still have active cancer, there is always the risk that this is an additional extension of that disease. But I wish you to also consider the possibility that this is part of an infectious process - not further cancer, which characteristically presents with pain and sensitivity. I hope that you will bring this to your doctors attention at the earliest time. Even an infection is a complication that can get out of hand if not addressed promptly.