Dog Yelps After Morning Poo?

Yes, as weird as this title says, this is exactly what has happened twice to my American Eskimo, Cloud.

Information:
Breed: American Eskimo.
Sex: Male
Neutered: Yes (done at 8 months)
Weight: 19.5 pounds.
Age: 10 months.

Note, Cloud came from a pet store. I am not a pet store advocate, however, I could not bear to see him suffer in the tiny cage as he had already spent 4 months in one, and decided to take him home with me and give him a life he deserved. The reason I am sharing this information is because I believe the fact that he came from a pet store, most likely meaning a backyard breeder, is clue enough that health issues are sure to pop up, which I was aware of when buying him.

Now onto my problem. It has only happened two times in the past couple months, however the second one just happened today and I am a tad concerned. Cloud went to the bathroom like normal (solid stool), and then as he was finished up he ran as fast as he could to me, yelping in terror. Now I'm not sure if he had a piece of poop stuck to his fur and was acting like a drama queen, or if he was acting out in serious pain. The other reason I am worried is occasionally I have seen him slip on his back legs pretty clumsily going up our stairs since he was 6 months old. It's only occasionally, and my mom believes it just to be the laminate flooring, however I am a firm believer that anything thats 'nothing' could be 'something'. For some reason, I believe that something may be wrong with his back legs or thighs or something along those lines, though I am unsure.

I am wanting to consult my vet on this, however I am bad with words and was hoping to get advice from here first to better explain to my vet my fears, and hoping they don't blow it off as 'nothing', even if it may be. I'd rather to be able to prove its nothing anyway, rather than the condition to worsen as time goes on without treatment.

Sorry about all my rambling, and sorry if this is truly nothing to worry about. I have tried google for something similar, but haven't come across anything like this before, and am hoping you can give me some insight.

Maybe he's a bit constipated. My basenjis sometimes do the same thing and if they have a piece dangling caught by a hair or something they scream the place down.

hi are you sure it is not just a crazy excitement when cloud comes running back to you, I would imagine if it were any type of pain he would maybe squeel as he passed the poo instead of afterwards, also if you are worried the slippy legs and this problem are related in some way, I feel quite certain if he had bad legs hips causing pain as he pooped he would walk backa bit slow and wobbly from it?
You know Cloud best and should seek a vets opinion if you feel something is not right, Going up and downstairs on laminate is probably not very easy for a small dog and yes he is maybe slipping x

I had a ridgeback with that. Prostate problem I think. He was not castrated though.

You don't seem too bad with words to me

I can understand your concern, especially if you are linking it to back leg problems. It could be that the position that he is forced to hold when pooping causes pain in one or both legs, but equally it could be something very trivial.

If he is fine in himself at the moment, then perhaps you can leave it until your next routine visit to the vet and then get them to check his hips and back legs.

I hope that there is nothing wrong, and well done you for rescuing this dog.

No idea, I just got home and he pooped just fine :3 Might be worth a shot.
I don't think its excitement because he has his ears back and tail tucked and is scared, he runs to my legs and sits on his butt. Thanks for your input
Hmm, I wonder if prostate problems are still possible even in neutered dogs? He wasn't done til 8 months
Well I tried to put a lot of thought into this, but I'm better at wording things when I have a computer monitor in front of me, not a real person staring at my face.
At 1 years old, March 16th, he goes in for his annual shots, and I believe thats also when they do his and Yuki's heartworm test. Perhaps it would be good timing to also ask them to do something like a hip test just to see if anything causes him pain?

Thank you all for your wonderful replies, I'm just an over-worried dog mom and can't stand the nervous thoughts of my pups suffering even the slightest bit.