Reoccuring Bumps On Legs, Redness, Stiffness, Filled With Pus, Painful. What Could This Be?

For the past four years I have had a reoccurring skin condition. They start out looking like zits, but swell to be about the size of a nickel or quarter. The area around the bump becomes very red, stiff, and the whole thing is sore to the touch. Instead of being filled with oil they are filled with a semi-thick white/ yellowish puss. Once I get the puss out, the bump starts flaking skin and will 9 times out of 10 result in a dark purple scar . I believe this condition originated during the summer of 2009 when I would go often times go to a friends apartment complex to swim. The condition started with a single bump on the lower portion of the back of my calf . It seems to be isolated to the back of my right leg. Over the past four years it has worked its way up my calf to the lower portion of the back of my upper leg. Each bump seems to last for about two weeks. Usually I will only have one bump at a time, but I believe the most I have had at once is three. These bumps itch really bad and bring excruciating pain for about four days when pressed, this can make it really hard to sit. They seem to appear about every two to three months. Thanks for the help