Why is my dog acting weird all of a sudden?

Literally "all of a sudden". She's usually very responsive and eager to do what I say, whether it is to come, sit, stay, fetch or anything. Ever since yesterday she is like a completely different dog. Whenever I tell her to do something, she just looks away and sometimes walks off. It's one thing that she won't do what I say, but the worst part is that she seems nervous and afraid of me, which puzzles me because I have never hit her in any way. She still wants to be active and if I go outside she wants to play, but she won't listen to me so I don't. There haven't been any changes in her diet and she does get exercise, it is truly a mistery to me why she's acting this way. I have checked if there is pain anywhere but she seems completely fine herself, she just kinda hates me lol. I'm annoyed because I'm the one that walks, feeds, trains, exercices and gives her affection. Has anything like this happened to anyone else? She is only 5 1/2 months old.