blood on bedding

Question
HI-I have 7 rats, 2 males and 5 females. I have 2 fairly large cages that I clean thoroughly once a week. I've established a sleeping area towards the top of the cage where I put a fresh towel every day or so. I've noticed on the females that there are times that it looks like there is blood on the towel. My girls are all under a year.
I worked in the Vet. field for 12 years. I had the opportunity to work with exotic, pocket pets, small animal, and some large. Surprisingly, I never worked with rats. I got my first girl and boy almost a year ago and fell in love with rats. I include this because I feel I have some basic knowledge of husbandry, care, etc. I know that the saliva can cause a red tinge, but it just seems to be more than that. I never noticed any bleeding either.
Any tips for a novice?
Also, I have provided a litter box for my ratties. I use a different material than their regular bedding. I leave a little of the dirty litter on each cleaning. They still just eliminate all over the cage, including their bedding. Any advice?

Answer
Is it just spatter or a pool?

Spattering is generally caused by sneezing or grooming at night, when they flick excess porph off of themselves; etc... a pool is probably blood and an injury. If you haven't seen any bleeding or injuries; be sure to check toenails, ears, and the inside of their mouths (good luck with that); and if nothing can be found --- it's porph. Have you noticed any sneezing or coughing fits?