Luckily, I'm not writing another post about our dog because things are worse. In fact, he's acted completely fine today. In the morning, I noticed that he was blinking a bit more than normal, but even that has gone away throughout the day. His behavior is entirely normal now. Nothing seems wrong.
It's such a relief after I was so worked up about it, and I really do feel so great today just because he's doing better. I'm still petting and cuddling him more than is typical, but it's out of relief and not out of worry or fear.
We're still not really sure what happened. He tried to go after a stray cat that was in our backyard the day he started having problems with his eyes, but my mom and sister were there and got him away from the cat. Neither one of them saw our dog and the cat actually touch each other. Plus, I noticed him acting off a day before that (I'm also wondering if that was part of what lead to him going after the cat), although no one noticed anything wrong with his eye then. And despite not being able to open his eyes, noting actually looked like it was wrong with them.
I don't think we'll ever actually know what it was that happened for sure. Not to judge a dog's reaction to pain, but a lot of his yelping the other day seemed like he was in more pain than just what an eye infection would cause. I just don't know.
At least he's feeling better now.