[Help] Dog won't go on walks, legs seem a little shaky but otherwise he's energetic and able to play. Any thoughts?

My dog's been acting a little weird since yesterday morning and I wanted to see if anyone has thoughts or ideas as to what's up here. He's a cavalier spaniel poodle mix, weighs about 20 lbs.So the main thing is he's been really unwilling to go on walks. He'll go outside, but after about 30 feet he stops and refuses to go any further. Sometimes he sits. He'll walk back to the apartment, but won't go any further than that. His back legs are shaking, sometimes more than others. Not sure what to make of that.This started yesterday (Monday) morning. He wasn't willing to go on a walk during the day yesterday at all, but I actually got him to go a decent ways late last night. He stopped about 30 feet from our apartment and looked back, but I picked him up and carried him a 10 feet or so and he enthusiastically went around the block with me before stopping and sitting. But I'm seeing the same thing so far today, where he hasn't wanted to walk further than 30 feet from the apartment. He's still been energetically playful this whole time as well, running and fetching and standing on his hind legs.On Saturday, I took him in for a haircut, after which he was feeling pretty fine. Sunday night I fed him some pork scraps, and I was worried yesterday that might have had something to do with his acting strange.At this point, I'm not really sure what to do. He seems to be doing fine overall, and is acting normal, but won't go on walks very readily. Any idea what's up in this case? Should I take him to a vet? via /r/dogs http://ift.tt/2rJFU0x