Why Does My Skin Bruise Easily?

For the past ten years, I've noticed red marks in my skin left by things that are pressed against my skin. These marks would last for a few hous then disappear. The condition seems to be worsening. Two days ago I sat in a metal park bench the had holes in it. Yesterday my family noticed red pattern left by the seat. Two days later, the pattern remains on my back. What is the likely cause and treatment?