Pain near armpit?

I’m breastfeeding and then supplementing with formula because I don’t make enough milk for my boy. Since yesterday I’ve had the pain in my armpit/upper boob area on the left side. First it hurt to press on it and now it just hurts all the time. I can’t tell if it’s a clogged duct because I don’t feel any lumps. I’ve tried massaging the area while nursing, as well as pumping, and it’s still super painful! No redness of the area either. Does anyone know what this could be?