You need to take her to the vet. It could be something fairly simple like an impacted anal gland - or not. Better to be safe than sorry. :-)
Here ya go! Send her this website. May she'll believe it - or not! :-)
http://www.marvistavet.com/html/body_anal_sacs.html
That was EXCELLENT advice. Couldn't say it any better myself, so I won't even try. :-)