Sore lip?

[ella777]

A few months ago, my mums friend had to move and couldnt take her fish. She had 2 platies and 1 guppy. They were in a tiny tank with a small plastic plant. I decided to keep them and they were very happy in a larger tank. I only have my female platy left now. But I was wondering, her bottom lip looks curved. I think it could be because she was glass surfing in her old tank so much, that her lip started getting sore. Shes thriving now, but does anyone know what might've caused this?
Thank you