Rust or Not?

Hello! I am posting a picture of, I think, rust but maybe not. I bought my 68 Dart about 6 months ago. It has a few small, pencil erasure and smaller bubbles just above the rear wheel wells on both sides. They are hard when I push on them with no give and there has been no change in them as far as I can tell. The car was painted a few years ago and the seller told me it was the paint process that caused it and not rust. I'm not so sure. Has anyone seen this and it was not rust, and what would be the best action at this point. I am not a body man but I believe I could fix this. Humm..... Any thoughts on the matter? Thanks. 20191026_135503_1572134453422_resized (2).jpg