My puppy doesn't want to walk anymore?

I have a 17 week old (4 months) old rottweiler and within the past 2 days he hasn't wanted to go for a walk. He used to love them, and had no problems. He still has all this energy, but doesn't want to go walking. when i take him outside with his leash on he will stop after a few feet, then he will resist as I slightly tug him. Today I had to pull him all the way to the street before he started walking, and he still resisted for about 15 more feet