How To Cure Reddish Skin Lesion Causing Itchiness On Forearms And Shin?

I have a skin lesion that appeared overnight on my forearm. It is red, purplish and blotchy. It is not raised but did itch in the middle of the night and then stopped itching by morning and day. I sometimes have lesions or rashes that appear on the tops of my forearms and shins. I also have rheumatoid arthritis and wondering if it is related or something else.