I don't work much with fondant but it may be air trapped under the fondant. Moisture from the cake. Stick a small pin or toothpick at a angle into fondant & let air escape, might help. maybe someone else will have a better idea.
Too late for your problem now, but I see that Betty's tip worked for you, good. It was almost certainly air trapped under the fondant. Make sure that you have smoothed your fondant thoroughly over your crumb coated cake and that it has stuck to it everywhere. If you have left a small part 'loose', then air can expand in it.