What Causes Small And Itchy Red Bumps On The Body?

I have small, itchy, raised red bumps, scattered loosely on my upper body. They are mildly itchy until you scratch a little, and then itch more intensely. They are only my lower extremities also, but only in a small area on the inside of my thigh above my knee. On the upper body, they seem to be mostly located on my anterior thorasic and abdomen (there are some on my back, but fewer in number), from my neck down to my pubic area, and the insides of my arms, axillary to wrist. They are not on my hands or feet. Approx 5 days ago I was swimming in a lake in Montana, and had a skin reaction about 1 hr after I got out that caused extremely uncomfortable itching all over my upper body. There was no visible rash or changes to skin color at that time. My family had all been swimming in the same water and did not have a reaction at all, but I was the only one to go for an extended swim into the middle of the lake, where I unintentionally swam through a bed of lake kelp...