Oh
@Fuzzyhead A hug for you

I am sorry you have all these new symptoms to cope with. Yes it's scary and can get anyone down....very down. I've had a whole load of events -some new-ish and some the old ones that got worse - recently, all of a sudden. It isn't much fun, has been making me anxious myself, and very down as every day things have felt like they are a bit "different" this last week....worse....
The big bad trouble with this illness is, it's quite hard to tell sometimes if these awful relapses (which can as far as I have discovered, include new symptoms sometimes) -are just that -relapses which resolve....or if they are a whole new thing; another illness! And yes anxiety can go sky-high. Even worse when we are feeling so rotten, the last thing we want to or need to be doing is traipsing round doctors' offices and hospitals. That makes the anxiety 10X worse.
I think there are certain markers in a regular blood sample which if spotted by the lab, would prompt a doctor to send a patient for further tests; certain clues, let's say, to make them wonder if it was cancer. (though of course a definite diagnosis would call for more testing.) But those primary "clues" would most likely be there.
I suspect cytokines actually when there is a new flare up of symptoms, even maybe including some new ones. If someone with ME/CFS is battling another infection, or trying to fight one off, a new cytokine party starts up, which can cause havoc, and very likely some of the symptoms you noted.
I'd had a very slight gum infection, started 14 days ago. It's gone now, as I used herbal antibacterial mouthwashes, with salt regularly (spit out of course after a few minutes). It is now perfectly cleared. But oh boy, just dealing with that simple thing set up a storm raging through my body and knocked my ME/CFS right back to what felt like square one.; like it was in the first few days.
I thought yesterday actually.....cytokines were probably involved. I always feel really sick when an immune response happens....for any reason.
But going for the scan you mentioned might set your mind at rest re: cancer.