Listen to Jaybay. It could be anything from intestinal parasites such as coccidia or giardia, for example, or something more serious such as anal gland abscess, ulceration in gi tract, foreign body or more.
I would have responded sooner but we had a tragedy in our family.
Take him to the vet. Now.
Sounds like he got into and ate something that his tummy couldn't digest =(
He needs to go to the vet to make sure he is okay
My husband went out of town so I havn't been able to take him to the vet he is to big for me to take him by myself. But It has stoped. So I don't know if I still need to take him or not.
At least bring a fresh stool sample to the vet for analysis. Ask the vet to perform a standard fecal test and a ELISA test for giardia. Your vet can treat your dog with medication even if the dog is not brought to the vet, as long as he is acting normally in every other way. However if he is sick and/or no parasites are found than he must be taken to the vet for an exam ASAP.