Most of my snakes leave a little streak of blood from their vents after shedding. I wish I knew what caused it--I've heard rubbing of the vent area, but it happens even if I pull the skin off before they get to their vents. At any rate, it's pretty normal.

The skin folds sound like the result of mid-winter under-eating. But -What's Wrong With My Snake- also says that sometimes that indicates health problems. Can you take a pic?