What Causes Itchy Red Spot On The Foot After Bitten By Some Insect?

Hi,

Twice now I have been bitten by something, once about a month ago and again two days ago. I do not feel the intial bite, but all of a sudden i have a really itchy spot and when I looked down on the front of my foot was a 1/2 inch size red welt with a juicy clear head on it. It was extremely itchy and I scratched it unintentionally breaking the head. The next day the whole red welt was hard and still bright red, today it has darkened and the hardness has disappeared. The one I had about a month ago left a faint red scare or mark. I don't know what it was or what it could be. Any ideas?