This occasionally occurs with breast surgery and is similar to a condition called Mondor's disease. It may be due to thrombosed or fibrotic lympatic or vascular structures.
Time and massage are probably your best options. It is not unusual for this to be most prominent at 3 weeks after surgery, remain stable for an addtional 3 weeks and then slowly subside after that.
Some physcians recommend anti-inflammatory meds during this period while others strongly recommend avouding these meds. Discuss this with your surgeon.

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