Hmm. Your vet would probably have figured it out already if this were it, but ... where on the face are the lumps? Is there any chance Chet could have a dental infection? My Eugenides has pretty severe dental problems (surgery has been postponed until other issues are under control enough for anesthesia), and a few months back he developed a weird lump on his cheek that turned out to be a dental abscess. (It emerged in a matter of hours, very alarming.)
My vet's office installed my cat's first Libre, but I do them myself now. We have some good information about using them over here:
Getting Started With Continuous Glucose Monitors (CGMs) for Diabetic Cats
Also, this guide that was put together by a diabetic pets Facebook group is extremely helpful:
Rise 360
If you do have your vet put it on, I highly recommend that you ask them
not to use surgical glue. Nearly every vet's office I've heard of does this, but it has been shown to a) irritate delicate feline skin and b) sometimes "flood" the sensor, making it less reliable. Gen and I had both experiences. If they're concerned about Simon pulling it off (which is valid), it works much better to wrap a bandage around it to hold in place ... the links above go into more detail about coverings. We also recently had a thread about that:
Seeking shirt recommendation for FreeStyle Libre
I do also recommend getting your vet to prescribe you multiple sensors/an "unlimited" prescription if possible, because the applicators are unfortunately single use and if yours gets triggered without securely attaching to the cat, you suddenly have a completely useless, overpriced piece of plastic and microchip on your hands. The company will generally send you a free replacement (as long as you don't tell them it's for a cat), but that's not an instantaneous process!
If you have specific questions about Libre application,
@Staci & Ivy,
@Jessica & Indy and I all use them (as does
@Lauren & Esse, but I haven't seen her around in awhile). Staci is particularly a pro with them!
Yeah, vets are like primary care doctors ... they have broad expertise, and sometimes with a more specialized topic like diabetes they're a bit at sea. My vet is really good, but this board has been immensely helpful to me (as has some independent research I've done lately, but not everyone wants to go digging through scientific journals, understandably)!