Frustrated ballet dancer?

I have 6 adult cats total, 4 inside, 2 inside/outside, all rescues as kittens. The tailless Manx mix female has started doing this weird thing the past couple months. She'll be curled up on one of the beds (we have several in the living room) and all of a sudden she sits straight up, does that staring at some imaginary who knows what, licks her butt (like she has an itch) 2 or 3 times, gets up on her hind legs and twirls around once or twice, stops, licks her butt again, twirls again, etc 2 or 3 more times. ????????
Jane

Check her rear.

I have several times. Nothing...but I keep checking because a little voice keeps telling me that there must be something there, but no, clean as a whistle. A little part of me keeps hoping that maybe just one of the cats isn't mentally unbalanced but so far no luck on that either.