What Causes Pressure In Breasts After Undergoing Lumpectomy?

I had a lumpectomy in my right breast 7 weeks ago. I was experiencing some sensations of breast "filling" (almost like let-down) in both breasts periodically. I have suddenly experienced an engorgement feeling (pressure) in BOTH breasts and burning on my chest between the breasts. I noticed these symptoms the day after beginning tamoxifen. Could this be a side-effect of the med or is this most likely nerve pain from surgery?