What Causes Knees And Legs Skin Blotching With Redness?

My skin has been turning red and blotchy when I shower or when tan in a tanning booth. This mainly happens on my knees and legs. No one I know can tell me why this is happening because no one I know has this problem which makes it embarrassing. I've gone to a dermatologist and he had a bunch of blood work done but everything came back ok. What's going on and what I can do about it?