Thanks girls .

I'm glad to hear that it's probably just the age. I felt like I'd 'broken' DS or something. It's hard seeing my usually happy boy so miserable all of a sudden. It happened again today at MG - as soon as he saw the unfamiliar house and unfamiliar people he was clinging to me and would not let me put him down for ages... but then he suddenly got over it and was happy wandering around without me nearby. Strange little people they are.
Also, thinking about it, DS has recently become an even bigger mummy's boy than usual recently. He keeps pushing DH away and diving at me for cuddles. It's nice for me, but DH feels pretty bad - poor DH.