Crying before and after using litter box

Hi there - I've had a good look though the archives, but thought I
would try this on some of the regular contributors.

I have a 6 year old female Burmilla, ex show cat (champion, no less !)
and breeding queen, but now a one cat indoor family (spayed). I've
been happily owned by my kitty for about 18 months and we've been
rubbing along just fine. She is noisy and loves attention, but no
more than any other Asian in the prime of life !

She's in perfect health - but recently I've noticed some quite strange
behaviour when she comes to use her litter box. The box is a large
style Littermaid and I use World's Best cat litter (and have done ever
since I had her).

Before she gets in the litter box she's started to go through a really
elaborate display, puffing up her tail and making deep and really loud
gutteral "calling" crys (sounds like a "wa-wa" rather than a "meow")
next to it. When she's done with the display, she just gets into the
box and does her business - no noise, no mess, no fuss, no discomfort.
Then she gets out, walks about a foot away from the box and repeats
the tail puffing and noise display. The vet has confirmed she's not
in pain or anything when urinating or defacating.

I've had cats all my life and never seen anything like it. It's like
she's advertising the fact she's using the box to me or something.
Any thoughts gratefully received ?