Kitty is drooling, sticking out tongue and licking mouth

Mickey is a about a year old, the little troublemaker escaped from my apartment and was missing for a month before he was located and I brought him home. He's been home less than a week, already taken him to the vet where luckily, they only said he had fleas; he was tested for parasites which came back negative. 

So now.... I've noticed that he has been sticking his tongue out, just the tip with drool and he's licking his mouth a lot. The vet checked his mouth and said she didn't see anything but I'm starting to worry. I had a cat who drooled/stuck out tongue and it turned out to be cancer. She had a noticeable bump as well. 

He also some how managed to injure his nose since he's come home though it seems to be healing nicely, though he does have a runny nose as well. 

Any thoughts??? Found it strange this all started after the vet visit.

Is he eating normally and using the litter box as normal?  My first thought is nausea.  Have you checked his teeth and gums to see if you see anything abnormal?

He seems to be eating normally, I gave him some treats to see if he was struggling with hard food and he ate them just fine, same with his wet food but I noticed he's using his paw to dig out the food once its a certain point. 

Litter seems to be okay, he had diarrhea when we brought him home and the vet gave us probiotics to give him and the poop is hardening up day by day. 

I checked his mouth and it all seems pink and normal to me.

I'm stumped.  She's not doing this while grooming or during petting?

Drooling was occasional when we petting him. Now its all the time 

Interesting.  I think I'd take him back to the vet and ask them to do a thorough check of his gums, teeth, and mouth and ask for some bloodwork.