What Causes Pain And Swelling In Ankle Post Insect Bite?

I was bit, I believe, by an insect, possibly a black widow on my ankle. It has now been almost 5 days and my foot continues to swell dramatically accompanied with mild-severe pain. I went to the ER 3 days ago, at which point swelling was mild and was prescribed Cephalexin. Is it normal for swelling to continue this long after initial bite?