Yup. Soleilvie, you may hear lots of conflicting things about water changes in the future so here's a tip that will help you know which advice to listen to.
You can never have too many water changes.
A betta's best defense against anything is fresh water and lots of it. Someone probably told you that because your tank was cycled you didn't have to change the water much, but that's not true. With a cycled five gallon tank, I'd recommend you change 50% of the water one week and 100% of the water the next week, alternating like that. That is, follow that regimen when your fish is healthy again. For now, follow OFL's advice.
I'm really sorry your fish is so sick now but you're on the right track to being a good betta owner because you're seeking help for your little friend. Everyone here was once a first-time fish owner who made mistakes too, so don't feel bad. Hang in there.