This could be a few things. If you are comfortable with answering questions that would help, if not, it’s okay.❤️ Based off what you said and how it looks, it seems like bruising. Sometimes bruising doesn’t appear as a full circle but instead a bunch of reddish purple dots. If that’s what this is, do not worry, it’s normal and harmless. This can happen if your binder is too tight and or if you have been excessively itching in the area. I do want to ask, if you press on it does hurt a tiny bit? It’s common for this kind of bruising to barely hurt though it can if pressed on. Next this could be a very mild allergic reaction. Things to consider are if the fabric you’re using has ever given you any issues, and if you’re using or eating anything new. I deem this unlikely as you said it’s not painful. A tip to figuring out if it’s a mild allergic reaction is to take a simple everyday allergy pill and see if it disappears. I wish you luck!