Kids go through separation stages at about 9-10 months, when they are learning to walk so 12-13 months and then for some reason it often comes back around 16-18 months and then again at about 2 1/2. It almost seems like an every 9 month issue for some kids but when you think about it there is a pattern of development that goes along with it and periods of learning independence - crawling, walking, running, toileting are also fretful periods for some.

Just be consistent and give empathy but sort of act like nothing has changed - as in reassure the child that life really is the same. Attempts to console are usually not well received and I think it is partly because the child doesn't know what is wrong with them either.