For me I would start with my Primary Care Doctor explaining the symptoms. Don't jump to conclusions. Get a CBC and thyroid levels checked. Perhaps other tests such as urine and stool. I had all of your symptoms you stated and found my thyroid was overactive due to bruising from radiation treatments. No cancer.
A PET scan can be useful in finding areas of cancer concentration, however a simple ultrasound of the neck can offer up a lot of information.
Take it one step at a time but don't delay. I fully understand our urge to believe every malady is cancer after the journey we have all been on previous. But chances are it is not cancer. Can you keep us updated?