It could be tail biting from stress, or if there are any other fish in the tank with the
betta they may be fin nippers and cause this. It would be useful to know what your parameters are. What is the
ph level? It is better to have a stable ph than to have a fluctuating one because bettas can adjust to different ph levels. For example my sisters betta's ph is 8.2 and he seems to be fine. If you want more specific betta care, I would recommend the website betta fish.com which is a forum specifically for bettas and also has an emergency care sheet so more people could give you advice. More pics of the whole tank could give people a better idea of the size and if there are any sharp decoration piece that could be ripping its fins.
I don't have a betta personally but my sister does and she highly recommends the betta fish forum, and if you fill out the care sheet it is very helpful.
If it is
fin rot, one medication I recommend is
KanaPlex which helps cure it. I used it on my
guppy who had it. It does work and instructions are simple to follow. When adding the medication make sure to remove carbon filter cartridge and then put back in once treatment is done.
Hope this helps! I am happy to give more info if needed.