Beach I know that feeling!
DS1 started school in September (we're not in the UK, its just mornings) the month he turned 3. We were
when the teacher described a shy, reserved, little boy who never spoke. The total opposite of the little boy we knew at home! He also cried a lot and wanted me all the time 
He's getting better and is speaking more, but still remains very shy around adults. Again, not something we're used to at home.
DH and I feel bad about it because we were both extremely shy as children and were relieved when DS1 seemed not to have inherited that (he was sooo confident in playgrounds, making friends etc., until he was about 2.8ish)
We found the his shyness came on around the time DS2 was born, but also around the time DS1 started to better understand the local language (he's bilingual, but much stronger in English than the local language for the moment). Once he started to understand the local language, he understood that some children didn't want to play with him and so became quite withdrawn and afraid of trying to play with other children in case he got rejected.
Before he understood the language, he was quite oblivious to the "rejections" and often the other children gave in and let him play with them (picture a 2 year old determinedly following three 9 year old boys round the playground until they give in and let him join in
)
I also was afraid that he was the only little boy at school having these problems...until I got chatting to another parent who said that out of a class of 32 children, two thirds of them still regularly cry at school in January! So it seems totally normal at this age.