I think that it might just be excess fat. Ive had this problem from time to time as well and it is a little concerning at first but I think I just really over did it in the fat intake dept. when Itook my pill!!
Your description is a little vague, but one possibility is steatorrhea which basically is excessive fat in your stool. This could cause it to be frothy, "float" and smell a little foul. Eating things like Olestra or medications like Orlistat could cause steatorrhea. You could Google it to try and find more information.