I get stung quite a lot through the job. I find if they come up straight away then they are no problem. The worse ones are those that come up after a day or so.
Not being melodramatic, but get to your doctors ASAP. Ring for an urgent appointment, and tell the receptionist that you've got an infected insect bite. You need to be seen NOW. There is serious risk of septicaemia, and if untreated can cause you to lose your hand, or in the worse case, your life.
Don't pussy foot around with things like this – they can be serious. Ring the docs, and don't take no for an answer. You need a course of anti-biotics.