Several of my friends that have weaned had this happen and it was a clogged duct. I would send your doctor a mychart message or give them a call, mine has a protocol to follow about when they want to see me for that!
How did you wean? You could have a full milk duct. Try a warm bath and massaging it? I’ve been told wearing cabbage leaves can help dry up milk and also be soothing?
Is it red and tender? Maybe mastitis! I had mastitis with my first baby 11 years ago so my memory is a little foggy. Maybe some warm compresses might help. Id say see a dr tho!