And now he refuses to walk

Last Monday my puppy finished his distemper series and the vet said it was fine to take him out on leash walks. I've been talking him around the neighborhood before work every morning (15-20 min) and we've been going to the park after work for a little bit to walk around ever since, and he's loved it. He's been walking with a loose leash and checking out people and dogs and having a good time. Yesterday morning all of a sudden he decided he's done with walking. We went on our usual route and he wouldn't walk. My husband tried at lunch, no walking. I took him to the park after work (literally kicking and screaming, because apparently now he also hates his crate for the car) and he walked a little but had to be coaxed the entire time. I thought maybe he was just tired because he spent a lot of time out playing all weekend, but this morning he still refuses to walk. The only time he will is when we're heading back home, at which point he pulls on the leash like crazy so we stop every two feet to wait for him to stop pulling, take another two steps and repeat, so it takes forever to get back home. As soon as we re-enter the front yard he's happy as a clam, poking around, tail wagging. Nothing upsetting happened while we were out this weekend as far as I could tell. He was happily trotting along on his walks, no problem. When he doesn't go on his walks he has way too much energy and is harder to manage in the house, but I can't drag a dog down the street twice a day. What the heck is going on with him and how do I help him walk again?