What Causes Swollen And Bruised Skin?

Hi,I'm 19 years old and got bitten by what I believe to be a spider. Several days ago. It has seemed like a normal bite until today when it started looking like it had a bruise around it. It is still swollen. I was wondering if its something I should be worried about, its on the inside of my thigh so I was thinking my legs rubbing together might be affecting it? I also noticed a dry area of skin around it, almost like its drying up. Is that normal?