Constipation, inability to have a bowel movement, stool stuck on the back end, poor grooming habits on the rump and top of back are all underlying conditions of excessive weight. I understand why she would get upset especially if it is really itchy and hasn't been touched by her for a long time. Some cats will grumpy and lick or chew excessively while you clean, brush or scratch this doesn't always mean they are upset with you it just an overwhelming feeling to have the area touched. She may even benefit from a bath or soaking her lower half in an aloe and oatmeal type shampoo and conditioner. Check for matted hair and either work these out or shave them off as they are very irritating.

If she does tend to get constipated you can feed her cat lax daily or she may need to go on a stronger medication like lactulose and cisapride.

Weight issue are very hard to overcome in cats. Prescription diets are recommended when the weight becomes a physical problem which in her case has. W/D canned is a great food to switch to. I actually use Nutro weight loss moist and dry food on my cats and they are doing wonderfully. Actually, I started them on this once they were all a year old. So many food are just too high in calories for the common indoor cat. I had tried Eukanuba restricted calorie in the past and they all actually started gaining weight so I went back to the nutro. So this might be something to consider if you aren't up to purchasing prescription foods. I would start brushing her twice daily and using baby wipes on her bottom. You may want to take her to a groomer to clip a potty path for her so that stool does not get stuck on her bottom.

Thank you

Theresa