The fact that you are passing stools that are little and round and shaped like pellets actually means that you are constipated because you are not eating any fiber. I have to say that the colour and consistency of stools sounds as if it is the direct result of your diet at present.
At the risk of sounding rude, which I really don't mean to be, why are you following this sort of diet in the first place? It is a very silly thing to do unless you really know what you are doing and are doing it under some sort of supervision.
A high protein diet can put your kidneys under extra strain and can also cause constipation due to the lack of fiber.
Your gastrointestinal system was designed to work with fiber and that is why a lot of people have problems with constipation when they just don't eat enough. Your body was not designed to be sustained with just protein alone, but also carbohydrates and fats, which is why we have specific enzymes in our stomach and other places in our GI system that will break down both fats, proteins and carbs.
Are you trying this kind of diet in order to lose weight by any chance? If so, don't. It really is a most unhealthy way in which to do it. To lose weight, you need to eat a mixture of both fats, proteins and carbs and plenty of fresh fruit and veg. You also need to ensure that you drink plenty of water to flush your system through and that you also take adequate exercise. If your energy output is greater than your energy input, then you will lose weight. It really is that simple.
I don't know whether you have been to see a Nutritionist of any kind, but if you did, I am sure that she/he would never advocate going on a protein diet solely, but would advocate a mix for health?
Unless you are a body builder in training for a competition, are a renal (kidney) patient, or are extremely catabolic following a surgical procedure and you need to help your healing process, then there is no reason at all that you would need this excessive protein intake.
As a final answer to your question. No, I do not think that you have cancer, but that your problem is directly related to your sole protein diet at present. Stop and I think that you will find your problems resolve.