Why Has My Dog’s Stool Shrank In Diameter?

I have an 11 week old Shih-tzu / poodle which was adopted from a rescue organization 3 weeks ago. Last week he was into the vet’s office for a stool check, which tested positive for round worms and was subsequently dewormed. The day after the dewormer was administered he passed what were obviously round worms, but none have been visible since.
He has been regularly exercising and playing, his energy level today seems to be normal, but he is crating himself a little more often than I’m used to seeing. He has eaten one meal today, and is due for his next in 15 minutes. His water intake has been a bit lower than usual but the weather has been more moderate this past 48 hours leading me to believe he’s not having trouble staying hydrated and is probably panting less.
My concern is that for the past 24 hours the diameter of his stool has been significantly smaller than normal and he is pooping more often. His stool has been a bit softer than normal, but I haven’t seen any blood or signs of anything he may have ingested that could be causing an obstruction or generally disturbing his normal G.I. functions. Unfortunately I’m not certain whether this is something he must see the vet about, or if this is a general symptom of constipation in a dog his age. This morning he was fed some plain yogurt and a couple of baby carrots in hopes of “moving” things along. He hasn’t eliminated very much since his first meal of the day but that is not unusual for him, he seems to need a few hours before he’s ready to go.
Any suggestions on what I should do at this point? Is this something I need to contact my veternarian about immediately?