L.T.
I had the same exact symptoms and it was a plugged milk duct. I would think your general practitioner should be able to confirm this and I would definitely contact him/her given your medical history just in case. For me I just made sure to nurse/pump as much as possible on the affected breast in order to dislodge the plug. Apply warm compresses and massage will help too. If you don't get the plug out it could lead to mastitis and you could then need antibiotics. My milk supply did go down in the affected breast but eventually returned once I recovered. Good luck and good job for being able to nurse this long.