Bruised nipple?

This morning when feeding the baby I noticed my left nipple is very painful. I tried relatching the baby several times but that didn’t seem to help and it only posed her off. I ended up just switching her to the other side. That’s when I noticed on the underside of my nipple is a purple line almost like a bruise. It’s hard to see unless your moving it around and looking for it. I don’t remember anything significant happening though. It’s been too painful to feed her from this side and even if I try and let her I don’t think anything is coming out because it makes me so tense. I’m not sure what I should do. I’m worried if I keep neglecting that side it will hurt my supply.