I can tell you that the same thing happened to my little guy.
He eye was really gookie every morning when he woke up.
It turned out that he had scratched his Cornea. My Vet used a black light (wand) and sure enough - you could see a scratch in his eye. I have never heard of dogs getting Pink eye - but I wouldn't rule it out either.....
Boxer's are such Ram Rods anyway - they just barrel in to things all the time - head first!
I hope you can find out what it is!