I think you should take it to a doctor. It could be half a dozen things. Rashes can take on many forms depending on the person's reaction and the source that's caused it. Are you sure it isn't poison ivy? Have you eaten, drank or come in physical contact with something new you may be allergic to? Some people are allergic to common things one wouldn't necessarily think they could be allergic to. I have even heard of people being allergic to sunlight, ice, cold, strawberries, nuts, chocolate, mold, mildew, dust and even know a person who literally breaks out from stress. I doubt it is spiders, it could be mites, but few spiders congregate in groups, therefore rarely attack in numbers. They don't tend to bite multiple times either unless they are trapped between you and your clothes or bed covers. There are certain forms of staph bacteria which can cause bumps, rashes and lesions, too. To be sure what it is and how best to treat it, you need to see a professional ...a doctor.
