Why is my puppy being such a little brat lately?

He is about 5 months old this week and lately he has been a little brat. He will just stare at me and bark like crazy, start running up to me and pulling at my clothes and then running away. When we have food he will NOT leave us alone. And when he does something he isn't suppose to and i say "No" he will like snap at my fingers. He isn't hungry or sick, and i do take him for daily walks and play fetch with him. It just started happening about 2 days ago. Why is he acting like this?