Can cancer return after you've had a unilateral mastectomy; chemo; Herception, inhibitor?

Mine was single side mastectomy as well and had chemo and now Tamoxifen. At my 2 year ultrasound and mammogram they said I had a cyst at 12 o'clock but nothing to worry about. I looked back on my original tests and it was present when I had the tumor in the other breast. I did say the surgeon could take both if he wanted but he wouldnt hear of it. Now every year I have to go through the mammogram thing and worry that cyst will grow.