So confused

I was dx with microinvasive ductal carcinoma with associated DCIS in Oct 2011. One lymphnode had micrometastasis - less than 1mm. Stage 1B. I had lumpectomy, chemotherapy, radiation, and one year of Herceptin. From everything I was told, there was a very low chance of recurrence bc it was caught so early and I completed aggressive treatment, chances in the single digits. 7 months after completing Herceptin, they found DCIS after core biopsy (birads 4 on MRI done only bc of highly dense breast tissue). I have been told that I now need mastectomy. What confuses me is how an invasive cancer can recur as DCIS??? I have searched the literature for any information on having DCIS after treatment for invasive cancer and cant find one article. All I can find is information about recurrence of DCIS? My worry is that initially I was dx with DCIS only and thought I would only need surgery. After the surgery they said it was invasive and I would need chemo. i am worried the same thing will hapoen this time. I thought someone may have had a similar experience and might find my post and enlighten me.