red sore patch on back of my cat's neck

Fire_Fox

IMO get registered with a vet now, some will prescribe you worming or flea treatment without seeing the pet if you are already registered. Plus you are on their books if you ever need the emergency vet at night.

Mrs_Z

Could it be a ringworm infection?

krlyr

I would perhaps consider a hot spot (often allergy related - any new food/treats you've been feeding? Could be a new cleaning product you've used in the house, washing powder, new plant in the garden or certain pollens in their air, etc) or possibly ringworm, though that tends to be dry rather than wet. Often recognised by the raised ring around the edge, hence the name (though it's actually a fungal infection rather than a worm, so worming products won't treat it)
Another possibility could be a grass seed that's worked its way into the skin and caused an infection, all the seeds are coming off the long grass and my dogs' coats are covered in them, all it takes it for one to even just pierce the skin and an infection could set in. Often they get imbedded before the owner even notices. Or in a similar vein, a tick bite could cause a reaction even after the tick drops off.
Multiple things it could be so a vet would be your best bet for a proper diagnostic.