Not being a medical professional, but a Medical Librarian who gives referrals, but not medical advice, I'll answer your question to the best of my ability. You have not said where the itchy rash is located. There are a number of medical professionals in this community as well, so if you want to get some more professional advice you'd have to answer that question. However the first thing I'd do (and you've said post-op, so I'm assuming you've had either a lumpectomy or mastectomy) is to contact your breast surgeon and see what they advise and take it from there. If you're already permitted to take showers, then perhaps a shower that's not hot may be a good start with no soap or irritants (no cleansers in the chest area) with tepid water, but that's all I'd say without an M.D. or oncology nurse advising you and oh, don't rough up that area with a vigorous towel dry-mersely wrap the towel around you for a few minutes to let it absorb the water.
Call your doc asap for advice.