This patient support community is for discussions relating to breast cancer, biopsy, genetics, chemotherapy, hormone therapy, lumps, lumpectomy, lymph node dissection, lymphedema, mammograms, mastectomy, radiation therapy, reconstruction, and self exams.
Any new lump or similar instance in an arm with lymphedema should indeed be evaluated as soon as possible. This could be a localized collection of lymph causing lymphatic ducts to dilate. If left for a long time, this can get infected and would need drainage and antibiotic treatment.
There are also some form of malignancy that can develop from lymphedemic sites called lymphangiosarcoma, but this condition is very very rare. To be on the safe side, it is best to have this evaluated as soon as feasible.
Regards and God bless.