It is unlikely that this is cancer since it was sudden onset and it does not fit the description.
It might not have anything to do with strep but you don't have to have a fever at all to be carrying strep, or it can present with only sore throat in some causes.
It does not fit the description of an STD.
Allergic reaction fist more
But there are other skin problems that can form here, including reaction to an inflammatory process such as autoimmune
reactions, infection, and it should be noted that fungus or yeast can present with only red patches on occasion.
As far as cancer, squamous cell is the only lesion close to this description....but again would not come up suddenly.
Given that you have myalgias...core muscles, a lab called a sed rate should really also be done, to rule out infection, auto-immune disease, cancer, etc. because if that lab is normal or low you have none of these.
good luck again