It doesn't show up well in the picture, but the blood vessels in the flesh are red?

Sometimes, my skin gets red like this when I stay in a warm place for a long time or use a hot pack. Does anyone know why this happens? I've seen it when I've been in a hot place or something, but I haven't warmed this area up or anything recently. The picture doesn't come out well, so I think I should describe it well, but I'm not good at expressing myself.