Help!

I have a weird rash thing on my arm and it just came up from no where and it HURTS! I’m 7 weeks and on zofan and Pepcid... anyone experience this as a side effect?? I haven’t changed my soap or lotion or laundry detergent so I have no idea what would have caused it. Any help on how to get rid of it or what might have caused it. It’s starting to spread so I’m a bit worried