Cockatiel being scared of nothing and angry for no reason?

A few months ago I got a cockatiel, and recently shes been having some weird behavior. Out of nowhere she'll tense up and flap her wings wildly like shes scared. She also just starts getting angry at things for no reason, and I don't know if i should be concerned about this behavior.

Also, do cockatiels whistle only when they're happy? She's been making whistling sounds lately and they seem to be more for attention or when she's scared/angry.

I might be worrying over nothing and its like normal bird puberty behavior, but this is my first experience with a pet thats not aquatic so