Help! my betta fish “sleeps” on the bottom on his tank?

My betta’s water was changed yesterday. Since the change, I noticed that the end of his tail fin looks like it was ruined. It looks reddish in the area (like it’s bleeding). I wasn’t the one who cleaned the tank, so I don’t know how it happened. Now, my betta just stays at the bottom of his tank. He comes up for air, maybe swims around a little, and then comes back down again. He was never usually like this. He used to be energetic, jumps for his food, and glares at almost anything. After yesterday, he’s lazy-like and doesn’t glare anymore. He used to rush to the top of the water to get his food, but now he just stays in the bottom while his food still on the top. I looked for signs of a disease, but overall he looks the same (except for that one fin).
Please answer my question! He may be just a fish, but I love him dearly. Thank you.