dear jane,
this is only my two cents. do not make any decisions based on my insane ramblings. i have neither examined or even met you in person. the best advice is that which is given by your doctor who i hope has taken a thorough history and physical and has amassed a database of blood work and other studies on you over the years.
there aren't false positives. however, there are techs who are just learning to do PET scans and screw it up. false positives? that's a nice way to put it. sounds a lot better than "screwed it up." if the tech was not incompetent, another explanation is that you received too large a dose of FDG (the radioactive sugar they injected 45 min before the test). the last possibility is that you actually have cancer.
get a PET with a different company at a different location, and ask if their techs are certified (as of right now i believe it is still not a requirement to be certified but this is changing). if you can get a scan at a place with a machine that is both a PET and a CT scanner, that is even better.
have you had a blood test called a beta 2 microglobulin, and if so, was it high? do you have any symptoms of lymphoma such as itching, fatigue, or palpable lymph nodes such as in your neck armpits or groin?
I have my fingers crossed,
-Amnia