I have what are considered normal stools and then sometimes I have what I call "fluffy or cloud like" stools that are about 1" - 2" in length. Sometimes I have stools like this together in one sitting. Sometimes the "fluffy" stools are accompanied by a strong urge to use the bathroom immediately and sometimes it's a normal urge. They are always brown in color. On occassion I have constipation but I feel confident that I can attribute that to a lack of consumption of water. In the past I have also had movements where I have no control and it's the constitancy of a thick milkshake with a foul odor. I have only had about 4occurrences of that over the past 4 years.
The consistency could be from a lack of enough fiber in your diet. You could try increasing your fiber intake through the foods you eat, or you can try adding Metamucil or Citrucel to your diet to add bulk. If it happens only occasionally, then you might just be low on fiber here-and-there as compared with chronically low.
If this is causing you problems it's best to consult with your doctor to make sure it's nothing more serious. But if it's something that just comes and goes on the off day here or there I'd just try making sure you're eating a well-balanced diet. Foods like fast-food, high fat, preservative-laden, sugary, diet colas, etc. can cause these symptoms as well.
I'm currently 5 weeks into a gluten-free diet and find that if I don't get enough fiber I'll have a BM that starts out firm, moves to fluffy, and ends mushy. Consistency of stool has much to do with what we eat. So, look at your diet and see how you can tweak it to add more fiber - fruits, veggies, legumes, etc.