Nathan took Hyperion to the vet today. Apparently he was just very constipated. However, because his fur is so long, his poops were getting suck to his butt. So his butt became infected and inflamed. They gave us a tuna-flavored kitty laxative to make sure it doesn't happen again. We also have butt cream so the swelling will go down and an antibiotic.
Other than that he seems fine. He's even walking around with his tail held up again, which I take as a good sign.
Thanks, Jules, for referring us to the vet, Nathan said he was good.