The small poop may be a symptom of her cancer, but it can also be normal. The oldest hen in my flock, although very healthy in spite of no longer laying, occasionally has bouts of tiny, solid poops like that.
It may also be a symptom of dehydration. I suggest two things. Give her a probiotic for a few days. Found in the pharmacy in the digestive aids. The second is to give her high water content treats for a few days - melon, squash, tomatoes, etc. And if she seems to be avoiding the water, try putting a little sugar in it to get her in the habit of drinking again.