How much is too much exercise

We have a new puppy, as you know. He's 3 months old now. We are an active family, who likes to explore and two kids who like to play with him and carry him, which they aren't supposed to anymore as he's getting too big.
How much is too much exercise? What are signs/symptoms that you've walked your puppy too much.
My pup Koda is acting a bit more lethargic tonite, whimpering in his sleep and in wake mode now and then. He's itchy very bad, mostly at night. I am wondering too if he might have a very tiny thistle of sorts in his paw that I can't see, as he pulled his paw back really fast and turned his head sharply when I was feeling between the pads. I am taking him to the vet tomorrow depending on how he is doing come morning.
any advice, thoughts.
I hope he's just tired from all the activity he's had this last week and a half.
I feel like such a bad mom for taking him on so many adventures. My mother is visiting from a different province so we hve taken her many places, of course, puppy has to come, to little to leave at home alone.

Hmm, I don't know, but when his tongue is shaped like a bowl when he's panting, that means that he is thirsty.