I'm the farthest things from an S, but I get this in an extreme way. When I'm relaxed and people are stressed, I physically feel the stress they feel, usually in the form of tension/tightness or sadness. I've always thought it was my own thing, but lately I've realized it's exactly what the other people are feeling.
For example, 3 nights ago a friend of mine came over and we were hanging out in my room. I told her that I felt her tension in my back, shoulders, and a bit in the chest, then put my hands on her to comfort her. I think it's a little like what the Reiki weirdos do, but I haven't read enough about Reiki (nor do I care to).