Ask the vet if her anal glands need emptying. wash her rear end with mild salt water solution to ease any soreness. Reassure her that all will be OK. Don't panic.
Let us know how you get on.
Good luck
I agree with Annlinda that her anal glands may need expressed.
If the faeces are runny for a while, the gland secretions don't get forced out as they should be and can build up and impact. It can be very painful and uncomfortable, and the classic sign is dogs scooting around on their backsides trying to relieve the pain.
You will need to take her to your vet to get the glands expressed.
Well maybe if she's had diarrhea(sp?) then maybe her bum is sore? i raised some newborn kittens and they ended up with the runs and sore little bums and the vet said i could use Preparation H hemorrhoid ointment and put a little on their bums to sooth the soreness.