Answer:
No matter how little or how much you use me, you change me every month.
White fat and fluffy, May even look like a puppy. Seems heavy but is light, Floats about without a worry, But if it turns dark you have to hurry!
In buckles or lace, they help set the pace. The farther you go, the thinner they grow.
I bubble and laugh and spit water in your face. I am no lady, and I don't wear lace.
What is often returned, but never borrowed/
People take me on walks and I run and have fun. What am I?