I have a one-and-a-half year-old female cat. When she is playing with her toys, her tail gets fluffy and sometimes it may sway from side to side.
Does this mean she is happy, or becoming aggressive with her toys?
Cats fluff their tails in two distinct ways.
The classic "bottle brush" or "exclamation point" tail, fluffed from the tip inward, is an indication of fight-or-flight reaction -- fear and/or aggression.
However, when fluffed from the base of the tail outward, that's what I know as "play tail". You'll see it when cats are mock-wrestling with each other, or with trusted humans.