Last Tuesday, I noticed a tender knot right below my left calf muscle. It's palpable without flexing my foot and I can feel more of it when my foot is flexed. It's not really noticeably tender unless it's palpated. Last Tuesday (a week ago) the skin above it wasn't discolored at all, you could see it from the side but mostly it wasn't visibly noticeable. Well today, it's red-almost bruised looking. It's pretty firm and about 1.5 inches in diameter. I wouldn't be too concerned if it hadn't suddenly popped up and then changed appearance after 7 days. Any ideas?