i have extensive skin discoloration on my right side after bilateral mastectomy. What is is?

its so hard to say without a picture.  it may be bruising or if its bright blue it may be the dye used to look at a lymph node.  Ask your PS or GS

Photos would be very helpful and also knowing if you had any complications following your procedure such as bleeding or tissue compromise.  And your best source of information would be your surgeon since your surgeon knows exactly what may have happened during your procedure.

Three basic forms of breast reconstruction exist. You can use your own tissue, implants or a
combination of the previous two techniques.
Your own tissue can be used in the form of the DIEP flap, PAP flap, SGAP
flap or fat grafting. Implants can be
done in one stage or two stage. Two
stage reconstructions are started by placing expanders at the time of
mastectomy. Once they expanders are
placed they are able to be inflated as determined by wound healing. The final
time consists of combining any of the above techniques.

Your question is rather vague to get a good answer.  More details would be helpful.  How long post op are you.  Did you have radiation?  It could be anything from residual bruising or staining up to pigmentation from radiation or just thinning of the skin.  Go and see your surgeon and maybe he can add more too.