How to stop my overweight cat from dragging her butt after pooping?

This sounds suspicious that your cats anal glands may be full. The anal glands are located just inside the rectum at the 4 and 10 o'clock position and when they are full they can cause irritation resulting in the pet scooting across the carpet. So this is a common presentation. Typically the anal glands are expressed by pets when they go poop, but in obese or overweight pets this is difficult and can result in them not being expressed properly.

So first thing is to get the anal glands checked by a veterinarian to avoid having them rupture because of over-filling.

The second thing is to get your kitty on a low fat diet or weight loss routine, such as decreasing portions and limited extra calorie snacks. Here are my picks for weight loss foods if interested:

1) Hills Metabolic- by far the best product in my opinion for FAST (within 2 months approx. 79% of pets lose weight) weight loss and weight maintenance (prescription only)

2) Hill's W/D - effective and also can help regulate glucose to prevent diabetes (prescription only)

2) Hill's Low fat - over the counter, varying results