Hello and thanks for researching this very important question!
Inconsistent appetite and a tendency to eat "treats" only in older dogs can lead to a constipation problem.
There are also certain medical conditions of the spine and lower bowel that can make it difficult or impossible for stool to pass normally.
1) You can purchase a children's glycerin (1.2 gram) suppository to see if you can get her lower bowel moving. If this does not succeed within 2 hours (usually, within 1 hour), then do have her examined to make sure she is not sick.
2) You can add a teaspoon of canned pumpkin to each meal to increase the fiber content of her foods.
***If she does not normalize within a day or two, or if she continues to have this problem, please have her examined by a veterinarian who can evaluate her health to make sure there is not a more serious condition to blame.***
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You could try the laxative...if she vomits or has other signs of digestive discomfort (burping, gas, diarrhea), then you'd have to discontinue this. Canned foods can be used with a 1/4 teaspoon of metamucil powder...it is non-flavored and will help draw water nicely. You can try running her regular food through a food processor and mixing it with water to make a porridge, since this is the food she prefers.
Tapeworms more often cause the body to loose weight overall, and would not bloat the abdomen unless the population has reached an excessive level...if you have been treating her for them, I don't imagine this is the case. It is usual to give a few treatments over several weeks or months to be certain all the "seed" segments are gone.
Slower bowel function will back up her entire system and cause a bulge. An overweight condition can increase this process.
There are medical conditions that can make the belly look pear-shaped (Cushing's Disease, for one...an over-production of cortisol from the adrenal glands). This problem may have a medical root, so do remain vigilant and suspicious of this.
This is a good discussion about Cushing's Disease: