Cat shaking and licking lips.

My cat (11 months old) is a male cat, neutered, all shots, etc. He has always been healthy and happy. One day a few weeks ago he came in from outside and went to eat some food, smelled his food, ran away and started shaking his head and licking his lips, and also shaking his head when he licks his fur. We were sure he got something on it from how he was acting but he spends his days out in the fenced in back yard with no access to anything unusual. He has been back there all summer during the day and this just started. We don't use fertilizer or chemicals. This has continued so he has been to the vet and got blood work done showing everything is good, also had him put under and his mouth examined and lots of x-rays taken and his teeth look good as well. Its a long frustrating ordeal. He is still happy but this thing he does happens a lot. He acts like he just got poison in his mouth and is shaking his head back and forth to get the taste out. Do you have any idea what it is? We have some creeping charlie in the way back yard, but he doesn't eat it. Can that happen from him just walking on it? Also we have 2 grape tomato plants which I heard are bad for cats, he doesn't eat them but can he get like that from being around them? Otherwise we don't have a ton of weird plants, some rose bushes, hostas, and other bushes. Do you have any ideas? Our vet isn't sure and we have run out of ideas.
Thank you for your help.