Why does my dog wiggle her back end when she's resting?

Question

My dog, Trixie, always wiggles her bum side to side, and sometimes up and down when she is resting on the couch with people. Sometimes she'll be sleeping and all of a sudden she'll wake up and begin wiggling ger rear. She'll do it for up to 5 minutes, and other times just for a few seconds.

Does anyone know why she could be doing this?

She is 14 years old, and a Chihuahua/Beagle mix. She is very healthy and in pretty good shape other then epilepsy which she is medicated for.

This is her:

http://farm3.static.flickr.com/2353/2322524510_3b9f6921a2.jpg

Answer 1

A parasite maybe. Give her fresh carrot juice to get rid of worms and parasites. If she stops the wiggling, it was just that.  

Answer 2

Perhaps a little doggy yeast infection, they have wipes called F1 at Banfield pet hospital that help, sometimes you can also use an antifungal cream after wiping the area. Doesn't sound fun but check the area it will appear blackish if it is yeast. Sometimes commercial pet foods have to much sugar in them for sick or older dogs this can equal a problem with yeast.