Why Does My Cat Drag Her Bottom Along The Ground?

It could have worms.

She probably has worms, or she may have an impacted gland. Her condition should be assessed by a veterinarian. You would do her a great favor by getting this fixed soon! This is not "normal" behavior for a healthy cat.

The most common reason pets scoot is when their anal glands become full.  Anal glands are residual scent glands located at 4 and 8 o'clock around the anus.  They are supposed to excrete a liquid during defecation.  Sometimes they do not empty completely and become full.  If they are unable to release this liquid these glands can become impacted and infected.  This problem is more common in dogs.  Have your cat examined by your veterinarian to have these gland inspected.  
Your veterinarian will also check for wounds, abscesses, or trauma.  Rarely intestinal parasites can cause this behavior.

For my dog it turned out with age she no longer could 'clean-out' her anal glands (vessels near her bottom that catch residue from pooping). For most these vessels clean out by themselves but sometimes it can stop working and just keeps getting fuller. Hurts and is uncomfortable for the dog. So she drags her bottom trying to relieve the pressure. I would suggest taking your pet to the vet for a check-up or ask for her anal glands to be expressed which has a smaller service charge than a check-up.....Unfortunately if this is the problem I'm not aware of any other way of fixing it.

He/she can't clean her butt. If he/she could he/she would, but he/her body can't fold that far to clean it. In that case you need to clean it for he/her. The way you do that is by taking a warm wet cloth and wipe it around where the circle is under the tail. When done take a new non we cloth and dry where you wet it at first. When done hopefully it won't happen again or until a very long time. When so you may have to do it again or not. Happy cleaning!!! Haha funny joke!!! Haha

Could be a itch in the rectum area if left undetected will become infected.

It may be because of an irritation in her butt it's time for a vet. Good luck