It looks like tail biting to me. Each tail biter bites their tail a certain way (some can get in between the rays, others just chomp away at what they can reach), but I don't see any black edges on the fin. Fin rot usually looks more raggedy and frayed.
My betta used to be a terrible tail biter... shredded his tail almost weekly and I never found out why. I know he was a biter because I'd catch him in the act all the time. But he grew out of it and his tail looks much better now. He never got a secondary infection, even though he basically was vulnerable all the time.
Keep up with the water changes, use stress coat, and make sure your betta food is high in protein to help the fin regrow ASAP. I tried using AQ salt sometimes but I didn't see much of a difference with or without it. Pristine, warm water is the best for fin regrowth. :)