I noticed a lump in my breast a few days ago and now my breast is hot and painful. What should I do?
It sounds like you have a clogged milk duct that has become infected. This is common. Clogged ducts often happen in the first few weeks of breastfeeding, because your body is making more milk than your baby needs. For more information on what can cause plugged ducts, click here.
Use ice to decrease swelling! Gentle massage before and during breastfeeding/pumping can help loosen the clog. Use your fingers to massage all around the breast as if it were a bag of marbles but be sure to not press too firmly straight onto the lump. (Don’t forget the underside of your breasts!) Cool packs and light massage can help a lot, but the best way to avoid clogged milk ducts is to feed your baby early and often. Feeding by dangling your breast above your baby while you are kneeling, known as Dangle Feeding, is another great way to open up milk ducts.
If the clog is difficult to relieve, it can become a breast infection called mastitis. Signs of mastitis are warm, painful breasts along with fever or chills. Your breast might look glossy with red splotches. If you notice any of these symptoms, it is time to call your doctor. You may need a breastfeeding friendly antibiotic to get rid of the infection so you can be pain and clog free while nursing!
You may notice a decrease in milk flow because the swelling from the inflammation can press on the ducts, slowing or blocking the milk from flowing well. Continue nursing on demand and use both breasts.
It’s normal for a breastfeeding mom to experience a temporary dip in milk supply when sick, including with mastitis. This happens because her energy goes toward healing rather than milk production. If you're concerned, track diaper output. Take good care of yourself; your milk supply will rebound!
SOURCE: https://www.llli.org/breastfeeding-info/mastitis/
https://www.medela.us/breastfeeding/articles/managing-blocked-milk-ducts-and-treating-mastitis