What Causes Red Swollen Lumps On Legs, Arms And Face?

I have red swollen bumps just under the skin on various areas (legs, arms, elbows, wrists, face) that seem to be spreading. it looks as if there may be a whitish infected area of the red bump. they start on few, resemble a large spider bite, but then get miore red, swollen, and possible pussy looking. They then become itchy. when I scrath, they seem to burst and get smaller, but then more appear. what could this be? I am generally quite healthy and and never had anything like this. please help!