What Causes A Painful Itching Welt In Inner Thigh?

Hi. Yesterday I noticed a huge welt in my inner right thigh, next to my vaginal area. It itched and I felt some pain, which start after I began touching it when trying to figure out how it occurred. Today, I noticed that it greatly reduced sized and redness and is now turning blackish-brown, like a scab. I'm not sure if I should get immediate attention or not.