When did my little boy turn into a Moody little Monster?

My little 4 yr old boy has always been rambunctious (just like most boys at that age) but lately he has taken it to a whole other level! He's mood swings at times can be bipolar. One minute he is totally happy and the next minute, watch out. He is in pre k so I am just thinking that it is just from the big adjustment from being at home with his family every day to now having to be around all these new people. Most adults don't handle adjustments well and hell sometimes I don't like meeting new people, so I don't really blame him. The first week of school was rough (whether it was rougher on me or him that is still to be determined) but now he is better and there are only the occasional days where he cries when he gets dropped off but this new attitude has emerged. We try to be very encouraging but still let him know that their are behavioral boundaries that he knows he can't cross but kids jobs are to test you to the limit at times, right? All I can say is that at school he is an angel. The teachers rarely have to really get on to him (or redirect them as they refer to it) and for that I am thankful but at times I think he seems that he is struggling with the fact that he is the baby of the family but he is no longer a baby. And, believe me it is hard for me to believe that he isn't the baby anymore...So to this I let out a big sigh and say to myself that I guess it is a Stage...Oh, how I pray it is b/c if not Mommy is going to take a nice little vacation to the hotel with the nice padded walls!!