it might be nothing but since I am observant I came to notice a few things:
my son goes to daycare 5 days a week, and it's been for more than one year, still he feels uneasy on Monday and cried a while before calming down.
He is a little shy and reluctant to greet his teachers, neither to his classmates. As a contrast, i noticed a few times his classmates approached him and greet him by his name and hugged him. He won't refuse the good gestures but showed no reciprocity either.
Also one time at a science museum for toddlers, he tried to push his way to the front of a line but an older boy pushed him back, and his reaction was crying and keep pushing forward, I and the father of that boy intervened and separated them and no real harm incurred. But it seemed to me that my son's only expression is screaming and crying when he want something. I can understand that because that's the way he behaves at home and most time he gets what he wants by doing that, he came to realize it's an effective way that works for him, even though I constantly tried to teach him to express himself by words not by emotions.
We are ethnic Chinese and we speak Chinese to him at home, I am not sure if that contribute to his social awkwardness outside the house. His teachers, however, guaranteed us that my boy was doing just fine in term of communication at daycare.
As I said at the outset that it might be nothing and I was just overworried, however if there is something requiring actions, I'd love to hear it. After all, I did notice some boys and girls of his age presented better social skills.