Suggest Treatment For Black To Red Insect Bite Mark In Thighs

I had what I thought was an insect bite a few weeks ago - the site was very red and itchy. It has left a black and blue mark that is fading. Now I have a whole ring of red spots round the site. It is high up on my inner thigh. I do a lot of gardening but always wear long trousers and tights. What is causing this?