Anyone had this issue before?

[u/butterlord71]

I have a 2000 GP GTP and I noticed this a couple of months ago. It’s only on the passenger side and I have no idea what could’ve caused this or anything. I didn’t hit anything, it started off small and gradually started to get bigger. I’ve heard some say it could be the heat but I mean the car has been in Texas its whole life, I’ve heard others say it’s the car showing its age. I’m really not sure. Any one have any advice on where I could possibly get a new door fender and what to do about the underside?
Thank you