If my feces used to float, but now tend to sink in the toilet, what does that mean?

Your diet. It all depends on what you are eating. If you are eating foods with lots of fiber (which is a good thing!), you feces will be more dense & sink. Floating stools can be caused by dietary fat or by an increase in gas from the normal bacteria that are in your gut. If you are not absorbing your food properly, the bacteria try to digest it & produce gas. See doc if not better for evaluation.