Painful pull under left breast three months post-op. What could this be?

This is a non-dangerous process called Mondor’s cord. It is rare but can occur after breast surgery due to normal inflammation. The tissue becomes inflammed and can cause superficial veins to clot. This creates a painful “cord” and is usually best treated with warm compress and motrin. See your PS if the issue doesn’t resolve in a couple weeks or if you develop warmth or redness in the area.  

This is a blood clot in a tiny vein. It is fairly common. Woman often get this without any prior surgery. It will go away on its own.