Chihuahua acting oddly

My mother's chihuahua is usually really energetic and happy, but for the past day or so he's been acting really lethargic and doesn't answer to any calls, where he usually rushes straight to you.

He was perfectly fine one day, but she did say he was in a garage for a few hours because he wasn't allowed in someone's house.

He hasn't been throwing up that I've seen, but I don't think he's eating or drinking much. Any idea what's wrong, or any suggestions?

[Aleda M Cheng, D.V.M., C.V.A]

Misfits4me has given you some good suggestions.  

Another one that is possible is hyperthermia or heat stroke.  It may have been hot in the garage and the dog could get heat stroke stuck in a hot unventilated area.  Of course, your dog's illness may be unrelated to the garage episode.  If unrelated, there could be many possibilities, such as: back pain from disk disease, pain from other causes, such as a soft tissue injury, Lyme or other tick borne disease, dietary indiscretion causing GI discomfort and many more...

Your best bet is to take your mom's dog to the vet for evaluation and treatment.

[Misfits4Me]

I have a suggestion while your waiting on the Vet to answer. You need to have your mom call the owner of the garage. You need to find out if there were any chemicals, Anti-freeze, or rat/mouse bait that her dog would have had access to......And you need to do this quickly as these toxins can be fatal......