Clogged milk duct or mastitis??

The side of my left boob hurts and feels tender and sore. It started hurting a few hours ago. It hurts to lift my arm and I feel sick like I have the flu 😕 its doesn't look red and I don't feel any lumps. My son had been nursing every hour & I've been putting him on that breast since it started hurting. Which one is it & what can I do?? I feel horrible! Btw I breastfeed with a nipple shield