Betta Fish: Betta fish fin problem, Help!

I got my new betta a few weeks ago and recently I've noticed his fins seem to be a bit ragged in the upper back fin. He was fine for the first couple weeks but in the last few days I've noticed shredding. I've attached a picture of before and a few of after. They seem to be a bit shredded but just is one area. I'm wondering if its fin nipping or something else. I have a fake plant with softer leaves and a cave that I have sanded down in the tank so I doubt that its from decorations but im not sure. All water parameters are normal. I had this same problem with my previous betta and assumed it was fin nipping but I want to make sure. Its a 5 gallon bow front tank with heater and filter. I do water changes every week. If it is fin nipping, I'm not sure what to do.