That is very possible. I wouldn't feel the need to go rushing back to the vet unless things worsen markedly. The next thing to look at is food: some cats are sensitive to certain ingredients. For example, my girl doesn't do well with beef or guar gum - nothing wrong with either of these in principle, its just that she gets very soft when she has them. Other cats have a problem with potatoes, or pea protein, or even chicken. Look at the ingredients of what you're feeding - you may find a link between giving one particular food and him getting softer poop.Could just be you feeding some food which your kitty can't take? Perhaps rotate food?