I would have asked you some more questions about your antibiotics that you were taking at the time?and also all other prescribed medication that you are taking at the moment? ANY pain killers?
and has it occured for the first time in your life?
But according to your question it seems like you are having Fixed Drug Eruption
and the morphology of the rash
and the antibiotics you are on is supporting my diagnosis.
The good news is that the rash can be left alone if not bothering but the IMPORTANT thing to remember is to report it to your doctor who ever is taking care of you at the moment beacuse its like a drug allergy
and if you take the same medication the it can appear agin with the same medicine and even in a much worse pattern.
i hope i have made my point clear at this stage.
thanks and all the best.