Cancer can go anywhere, but it usually follows a certain path depending on the type of cancer. Tonsil cancer can spread to nearby structures, including bone, spread through the lymph system, blood, vein, and nerve line too, so I can't say it's not impossible, but probably not probable just in the lower back without other cancer infiltration first.

It does sound like cancer anxiety, but can be another medical symptom, and you did the right thing to see your doctor, he or she ordering a PET scan. I've had back trouble for 30 years, which different type things can aggravate it, including not being active, stress, even medications can cause spasms like when I would get epogen shots, and last May it was due to kidney stones, and thought I hurt my back in the gym, and needed laser surgery to blast them.

Good luck with everything.