Jack Russell not acting like himself

Question
I have a 3 year old J>R> who usually is a spitfire.  We live in Arizona and the temp is 110 degrees.  He is an indoor dog that loves the out doors but hates the heat.  It is almost a chore to get him to take walks in the early morn
or late even.  He is just not himself. when we play with him inside, he stops within the first few minutes and turns away disinterested.    I can't see or pick up on any other physical symptoms.  He just acts like a sad little dog.  Any ideas?  Could it just be the heat

Answer
If he's 3 and the heat hasn't bothered him before, it probably isn't the cause. Have you made any changes to your household lately? Has someone left for college? A new baby coming? Has your work schedule changed? Construction going on? Any significant change in the owners life directly affects how their dog is acting.
If there have been no changes, I strongly suggest you take him to the veterinarian for a blood test. There are many many different illnesses that can cause lethargy in a dog, with the most common one being Lyme disease at this time of year. (do you get Lyme in Arizona?). Don't wait any longer, most illnesses if caught early, can be treated successfully.  Best of luck to you!