So I guess it turns out I did have mastitis and now that's all cleared up. However the weird lump is still there. It's fixed to a rib bordering the sternum at about armpit level, which isn't really near breast tissue for me nowadays (sigh, lol). It feels like part of the rib, but it definitely hasn't always been there, it's not symmetrical with the other side, and it's visible beneath the skin. However that's only the case if you look really hard, and even then it's not weirdly visible. It looks like it could belong there. It sounds scary (to me) because it's not movable and it's hard to differentiate it from the rib, but it also seems like it could just be from costochondritis or something...maybe. I don't know. I'm going to tell myself that until I find out for sure what it is. I did have what I believed was costochondritis for a few months when I was pregnant, and the pain was right in the area where the lump is now, so it seems that it could be related.