Dog's poop??

[lovely13]

Recently my dogs poop has changed.  He is a beagle mix 35lb 10 yr old.  He recently started pooping in small amounts.  He will poop several times a day but sometimes there is only one piece of poop.  It looks smaller to me like the difference between a pencil and a straw.(it is much bigger than straw this is just an example) Another way to explain it is the circumference of poop is smaller.     I thought maybe he was constipated so I gave him pumpkin a few weeks ago and that did nothing.  2 days ago he had some runny poop it was a dark brown.  I have had him on rice and chicken the last 2 days.  Yesterday He pooped his normal small poop and then later poop some loose stool that was a very yellow color.  The only thing that has changed is his food. He has been on this food for 3 months.   I was feeding him taste of the wild and switched to a brand name victor.  It is very similar in protein and fat and in that is doesn't have all the added fillers. He has never had problems switching food before.     What could be the problem?  Today he also acted like his stomach was hurting and almost puked but didn't.  

[Margot49]

This could be a sign of liver or biliary disease.  When was the last time he had a Vet exam?  Due to his age, I would take him in for some blood work and take a fresh stool sample with you.