What do you mean by "fanny?" Are we talking the whole region, or just the female parts (vulva)? If it's the vulva, my suspicion would be her first season. You'll want to keep her separate from your males, if they are intact m(and I mean SEPARATE! The can, do, and will find ways to try to get to her).

But the fact that her personality is changing slightly would indicate a difference in hormones, which would tell me she is probably coming in.

PLEASE, make an appointment for a spay! The vet can advise you on a time frame, but generally, they like to wait till at least 3 weeks following the cycle (meaning after the bleeding has stopped), so you can probably make an appointment for about 6 weeks from now.