My tiny chihuahua yelps when having BM

Over the past two months, my four lb, 3 1/2 yo, female chihuahua yelps in pain when a having BM.  Her BM has become quite large during this time.  I began mixing canned pumpkin (fiber and water) and a tiny bit of mineral oil in with her Puppy Chow kibbles.  The stool has softened but is still quite large at start then reduces in size to normally tiny.  Today I noticed the anus is externally red.   She is constantly licking that area.   When she yelps, she pulls and spits out the BM.  The BM is soft, brown and normal looking.  The Vet could not express anything from her anal gland (but she is so small).  She drinks a lot of bottled water.  At first I thought it was the density of the stool but now that this has changed a little and I see a red and tender anus, I just don't know.  She is active and playful otherwise.  My two other female chihuahuas have no problem.  The Vet told me to change her diet, make sure she drinks plenty of water.  I see a change in the poop.  But it is inhumane to let it continue.  Help, please!!!

[Aleda M Cheng, D.V.M., C.V.A]

The anus and rectum are parts of the digestive system and therapeutic trials of different diets may be necessary to find the correct diet for you dog.  There arm many diets to choose from, including making your own, trying a gluten-free diet or a hypoallergenic diets.  

There could also be other causes that are responsible for painful defecation such as: an anal sphincter problem, pain in the spine that occurs when your pet postures to defecate, a lesion in the abdomen that may press on the rectum,  and many others.

Probiotics such as Prostora (available from your vet), and a human product: VSL-3 (available online) are effective in stablizing the gut and help to normalize defecation.  The dose for VSL-3 is 1/8 of a human dose.

Medication for pain, such as Metacam, Rimadyl and Tramadol may also help.