What Causes Itchy Rashes On Legs?

I have a very itchy rash on the back of both of my legs, after sitting outside two days ago - it was a warm spring day and I was surrounded by tree pollen. I sat outside again today and just showered - it s much worse after showering. There are several raised red bumps on both legs that feel like big bites or bad ingrown hairs. It is very itchy and the entire area is red. I m not noticing the rash anywhere else on my body. Help!