dog shakes and listless

Question
I have a Chihuahua/Jack Russell x male about 4 years old. A week ago I took him to the vet because he was shaking and it looked like he was in pain walking. The vet gave him a shot of anti inflammatory and a couple of hours later he seemed fine. Every day now for a week he displays these same symptoms for a hour or two, then for a couple of hours
he seems normal with the exception of wanting help getting on the bed or sofa, then for the rest of the day he is running around playing, bossing around the two females, and jumping up on the bed or sofa on his own. I'm just puzzled why he looks so bad for a couple of hours and is just his normal self the rest of day.

Answer
Did the vet give you any indication as to why he was that way the first time?
It's quite possible that he gets a bit too rambunctious with the other dogs and pulls a muscle in his leg or back. It is in the nature of any breed with Chihuahua in it to shake when they are hurt or upset.

If this is something that continues I would have a more thorough exam done on him and possible x-ray his legs to see if his knees are popping out of place. They sometimes do that yet pop back in but he might be upset when they pop out and then he is back to normal once they pop back in.
It's just an idea that you might want to swing by your vet if he keeps doing this. It is very common in small dogs and can happen at any age.
Let me know what you find out.