I Have A Bite On The Lower Part Of My

I have a bite on the lower part of my belly dont know what bit me I think maybe a spider of some sort. when it happened it started burning really bad and now it is really red all the way around several inches, the affected area looks like a big blister and some black I went to the Dr, but it was so swelled they couldnt tell what could have bit me, said it looked like a big blood blister, the burning is still there, b?c I am over weight my belly rubs my leg.. sometimes it gets a sharp pain in swollen area. What do you sujest?