Pain after breast reduction. Should I be worried? Is it Mondor's disease?

Without photos or personal examination, online consultants can only utilize the provided information and speculate. This is a poor substitute for seeing your surgeon and asking directly, but what you describe is certainly consistent with Mondor's thrombophlebitis, a tender, cord-like thrombophlebitis (literally a "clotted, inflamed vein") of a superficial epigastric vein that was cut and cauterized in the course of your breast reduction incisions. (BTW, Mondor's is not a "disease;" it is not something that you "catch" but a response to surgically cutting and sealing a blood vessel.)As you probably know, this is also seen with breast augmentation where an inframammary incision cuts the same vein. The vein is cauterized by the surgeon, and without blood flow, the vessel clots and that clot forms a tender cord-like structure noted when the patient raises her arms or rubs below her breast. The location is on the ribcage, usually towards the front rather than the side of the ribs, and towards the belly.But again, without an exam to rule out other possible causes, no one here should give you false reassurances. Go see your surgeon and put your mind to rest (or get treated properly if there is something else going on)! Best wishes! Dr. Tholen

A Mondor's cord is definitely a possibility after breast reduction, as an incision through any of the veins that traverse this area can lead to clotting ("thrombosis") of the vein. From what you describe and your time out from surgery, this is a strong possibility. You should of course see your surgeon to have him/her examine you, but if it is a Mondor cord it is a self-limiting and benign process, but sometimes takes several weeks to clear up. Treatments such as warm compresses, ibuprofen, massage and ensuring you do not wear anything that presses over the area directly can help with the pain, but check with your surgeon before doing any of these things. 

Given your concerns, you will be best served seeking follow-up with your plastic surgeon ASAP;  online consultants,  despite your good description will not be able to provide you with an accurate enough diagnosis,  treatment recommendations, and/or reassurance to be truly helpful. Having said that, your description does sound consistent with Mondor's veins. Again, please follow up with your plastic surgeon. Best wishes for an outcome that you will be pleased with long-term.