I just wondered if you had told your trainer at puppy classes that you are using a collar ? If it is a decent trainer they will be horrified you are using it on a puppy!
If your puppy has a problem, whether it be rage or some other neurological issue, you do realise that the collar will do nothing to solve it in the long run don't you?
At such a young age, unless your puppy has a neurological problem which is making it behave like you describe, then it is purely a behavioural issue. Something that at 18 weeks can easily be ironed out by changing situations and the way you respond so it doesn't happen.
I do wonder if you are actually scared of your puppy ? If you are nervous of what you puppy will do next, these vibes are feeding into your puppy and making it feel unsure of itself.
Have you had you puppy checked by a vet for any pain issues - when you are patting it, picking it up, putting collar on ? Maybe a certain place you touch is causing pain ?
As others have said, resource guarding is not uncommon, it seems to be quite prevalent amongst spaniels. When I gave my lab a bone as a puppy she would have had my hand off if I tried to get it, so I stopped allowing her to have anything which was of high value as a puppy. I also worked on giving and taking toys off both my dogs from puppies and I still do now. I take the toy, praise them and give it back. As puppies I took the toy, praise and reward with a treat and give the toy back. When they realised this happened they would frequently sit with toy hanging out of mouth waiting for it to be taken so they could have a treat. If the bone is so precious don't allow him to have it, it's a simple answer!