My dog was spayed 2 months ago, and this appeared only where she was shaved.d

Hello!  There's so much I don't know here - When did it start to appear?  What were the post-op checks like?  What happened / did it look like after the rash first appeared?  How has it progressed since?  Is it itchy? Has any treatment been tried?  Why was the spey done and were there any abnormalities there?  What has been done so far to find out more?  I am in no position to say whether or how this might or might not relate to the spey.  Did the square shape of this area represent a dressing?    Sometimes when fur is cut short, the regrowth can be a itchy and I could imagine this leading to scratching and skin infection if not stopped.  There is also such a thing as a clipper rash.  Some parasites eg fleas (itchy) or demodex mites (not itchy) can proliferate when dogs are speyed because the immune system has more work to do at this time, although it would be unusual to form in such a straight square.  I think that the first step, if you haven't already, would be to show the rash to the vet, and have them assess it.  A sequence of tests (not of all which might be needed in any one dog) could help to get to the root of the problem if needed.  Regardless of the cause, the area also looks as though it may still benefit from appropriately prescribed medical treatment sooner, rather than later.

Here is an article from the blog about pyoderma, as it seems likely that there may be infection involved:   https://vethelpdirect.com/vetblog/2020/08/01/does-my-dog-have-a-skin-infection/