K.F.
This is common when you wean. I got them all the time. It sounds like a mastitis. It is a clog in the milk duct. You will have to massage it out. Press on and around the lump and massage it toward the nipple. Yes, you will be expressing some milk out and it will take a bit longer to stop the milk on that side. Pain is not good, so do what you need to do to stop the pain and get the lump to smooth out.
If you get a fever or feel like you've been "run over by a truck" or it bothers you too much, go to the doctor. You have an infection in your milk duct and need meds to get rid of it. Good luck. They aren't easy to get rid of and the worst part of weaning.