What Causes Painful, Hard, Red Knot On Areola?

i have a hard red knot on my left areola above the nipple. it is painful and warm to the touch, has a burning sensation. it just showed up yesterday, and i dont think its a skin infection of any sort and no injury. it is swelling and turning the skin red. there is no discharge of any sort...should i go to the ER? it hurts a lot!! help