Just a thought - my DS#2 has been bitten by a few different things in his time and our doctor seems to think that a few times they have been spiders.
The bite came up quite large, red, swollen and blistered. Apparently most spider bites dont hurt when the spider is actually biting.

The doctor cleaned the area and put a "pad" with betadine on it (kills germs and takes out the infection) then bandaged it. Had it changed every 2 days. Was on his lower leg/ankle area the first few times. Since then if he has had the same reaction i dont go to the doctor i just do the above myself and all is fine. I havent needed to bandage - i just put betadine on twice a day. Depending on the size of the bite i put betadine on then cover with a bandaid for the first day to make sure the betadine gets in.

Now, i am not saying this is what has bitten you, sounds similar and the treatment works for us.
I would get it checked cause these things can be horrid.
Good luck !