Ive had an alergic reaction and need help!?

Ok so I had an alergic reaction, unknown what though, I havn't eaten or drinken anything different, and there WAS a rash all over my body, it's only on my legs now though, it needs to be there for at least 3 days for me to see an alergist person. anything I can do to keep it there because I seriously need to see the person, I need it there for one more day