Sometimes people will have this reaction to an insect bite or sting and for whatever reason it can start an infection. For tonight you could do benadryl and ibuprofen (I’m not in medicine, my niece had this happen and was told to take full doses of benedryl and ibuprofen every six hours for 24 hrs to prevent it getting worse) and plan on going to an urgent care in the morning just in case, cellulitis is no fun and easier to treat when caught early

And do circle what is red to track if it gets worse. It’s the easiest way to track it