Rash at 4weeks!

Hi everyone, I got a BFP yesterday and have had this horrible rash for about 3 days now, I’m not sure what I’m allergic too but it’s so bad!!!!! It’s only this arm so far, I have an appointment tomorrow with my OB but if anyone’s had a rash in the beginning of pregnancy, please.. what was your outcome? Can my doctor provide me anything? & how long did it last?

It started on one spot but it’s so unbearable that I end up scratching and it’s spreading down to my arm now