I think i was bitten by a spider. a few days later, an itchy red rash appeared on my chest. allergy meds help but it comes right back. moved to my arm?

Not likely spider: Most insect bites cause local inflammatory reactions like itching and welts that are mild and resolve within a few hours without complications. Most spiders are harmless, only a few cause more severe reactions (i.e. Brown recluse) which present as redness and blistering, sometimes leading to a blue discoloration, and possible necrotic lesion. Symptoms days later not consistent with bite reaction.