My 9 year old golden retriever hasn't been himself today. When I woke up this morning, I found 2 round, hard, black balls by where he was sleeping. I assume it was feces, although it was odd since his never looks like that, he hasn't had an accident in years, and it was essentially scentless. Since then, he has been very lethargic all day, not interested in walks, hasn't eaten any of his food (he did eat a very small amount of mine), and gets up/walks around very gingerly. He has been drinking a good amount of water though, maybe a normal amount, maybe slightly more. He seemed perfectly fine until this morning. Any ideas what could be wrong?
Edit: I should also mention that he had a vet check-up about a week ago including a blood test, and they said everything looked normal.