I had lymph nodes taken out after I had breast cancer in my left breast. Why can't my blood pressure be taken on that side?

Good question! The theory is you could cause lymphedema by squeezing the lymph channels with a blood pressure cuff. However, there has never been a study to actually see if this happens, and i think it's more myth than fact. I tell patients with bilateral mastectomies it's ok to have their blood pressure taken, and no one has ever developed lymphedema from it. If you already have lymphedema, i would avoid.
Prevent arm swelling. Sometimes after surgery, it's harder for lymph fluid to drain out of your arm. If the fluid builds up, your arm swells ["lymphedema"]. Measuring blood pressure on your right arm can help prevent this swelling. Here's to your healing! other ways to prevent lymphedema can be found here: http://www.Breastcancer.Org/treatment/side_effects/lymphedema.Jsp.
Lymphedema risk. The risk of clinically significant lymphedema after full axillary node dissection is 20-25%.With the more common sentinel node biopsy procedure, less surgery and fewer nodes removed = significantly less risk but not 0.Nodal surgery and scarring may affect lymph channels draining fluid from the arm.The BP cuff increases pressure in the lymph channels causing stress to the system increasing le risk.
Lymphedema. You are at increased chance of developing lymphedema ( arm swelling since your lymphatics are not draining well). Taking blood pressure increases the pressure in adjacent vessels that can cause fluid built up in tissue. Take good care of your skin to avoid infections. Avoid getting IV or injections in that arm as well.
Avoid lymphedema. Image the lymph system as a lacy network of canals that help drain fluid out of the arm. Lymph node biopsy interferes with the drainage system. Anything that causes swelling in your arm, including 'back pressure' of a blood pressure cuff can cause swelling in the lymph system in the armpit- and increase chance of chronic-permanent swelling in entire arm. Doesn't always happen, but it is possible.