Did I accidentally make cheese?
Yes, you did; although in a non-traditional manner :)  You created an acid-set cheese.  These are made by adding an acid (e.g., lemon juice or vinegar) after heating the milk (e.g., paneer) or by heating soured (acidified) milk (e.g., quark).

It appears that your microwave blast caused the proteins to combine into a single clump.  Normally, the heat is more gently applied.  From your picture, it seems to be most similar in texture to the curds used in a poutine.

Let us know how it tastes!