Lump 7 years after mastectomy

I had a double mastectomy 7 years ago. IDC, stage 1, grade 2. Tamoxifen for 5 years. Yesterday I found a pea sized lump in the opposite breast where the cancer was originally. I know the odds are minimal that this is cancer again, but what could it be? What does a lump feel like when you have implants?

It took several years, but I got to the point where I just didn't worry about this any more. Now, I've got that familiar panic while I wait to call the oncologist Monday morning.