Hi and welcome to petfish
This is most definitely a bacterial infection (which is what the fin rot is too). How often do you change the water in your bettas tank and how big is it??
It is best to cup your betta and do a 100% water change on the bowl/tank making sure that you match the new tank water ti the old tank water (and treat the water too
). Add 1 teaspoon of rock or aquarium salt per gallon and dissolve it fully into the water in the tank/bowl once each water change. Float the cup in the tank/bowl for half an hour to match the temperatures and put your betta back. If the bowl is 1 gallons or less, then 100% water changes in the suggested manner every day adding the salt aswell should help out with the bacterial inection.
If there is no improvement after a week or so, purchase some maracyn2, do another 100% water change and start treating your betta....remember, its not a good idea to change the water while treating your fish with any medication or you will dilute the medication rendering it useless (unless otherwise stated to do water changes during treatment on the packaging). usually very clean water and salt help out alot, but if it doesn't then medications are necessary