It could be a million things but none can be diagnosed or treated over the Internet. You need to find the money to take your puppy to the vet.
It is unlikely that the long walk caused a problem (in terms of over-exertion) unless he was unused to walking. However, who knows what he was exposed to on that walk. If he's never had vaccinations he is at a very high risk for a number of life-threatening diseases. Again, the vet is the only one who can diagnose your puppy.
Ruth & the 4 Legged Rescue Gang
Rafi the malaroo
Gio & Varda, the krazy kittens
....In my heart: