My Extremely Shy Guy?

So, I have been a bit worried about Finn lately. As a wee child, he was boisterous and outgoing, and then shortly after I became pregnant with Phoebe he became more shy; he started to become a bit timid when doing things without mom or dad, avoided conversation with strangers, and sometimes even avoided other children at the playground. I assumed it was a stage. A reaction to my pregnancy. Or just being two and a half. Or whatever. But something that he would soon outgrow.
As time has gone on, however, this "stage" has continued, and he has become even more shy. I would actually describe him as extremely shy in certain situations. So much so that it is in fact affecting his "success" in interacting with the outside world and he doesn't always act appropriately in social situations. Sometimes, if he feels uncomfortable he may run up and kick me or make a loud noise in someone's face, and I can tell it is because he is just not sure how to act. However, his extreme shyness seems to be somewhat random, where one moment he can't deal and acts completely inappropriately, and then the next he is an outgoing chatterbox and will strike up conversations with complete strangers.
Let me give you a couple examples to show you what I mean... We signed him up for a soccer class when Phoebe was just a few weeks old and he had 3 other friends in the class. I was sure he was going to love it. Instead, I found that each day, when asked to go with the group, he would cry, refuse to look at the instructor, and then sit down in the middle of the gym and scream for me. Needless to say, he didn't participate much. However, several times after class, he would wander up to the coach and start telling him these long stories and not stop talking. Go figure, right?
For the past couple months, we have had Finn in swim class with his best friend. Each day, he is reluctant to go over with the group and refuses to look the teacher in the eyes. The first day of class, he actually threw a bucket at the teachers face when he wasn't sure how to react. That was quite embarrassing for me. But, by the end of each class, he is splashing around and having a great time, but then 2 days later, when we are back, he once again doesn't want to get in or even look at the teacher. It is like we have to start over and warm up to the teacher every time.
He also refuses to go to the potty at school. Which is at his best friend's house. He will go if I go in with him, but not if I am gone. The whole idea of it overwhelms him. And some days we will go to the park and he will make a new friend and play for an hour. Other days he will come running to me if another child even looks at him.
Is this normal behavior for a 4 year old? I know that kids his age get moody and have "off" days, but this behavior is consistently inconsistent. If it is 'just' a phase, it is certainly a long one, because this first started almost two years ago.
It has crossed my mind to wonder if he could have some form of autism, but aside from these strange social interactions, he seems totally normal. He has many friends that he is not shy with at all, he likes to be social and interact... he just needs it to all be on his terms and I can't seem to get him to break out of his shell on a regular basis.
Has anyone dealt with something similar to this? Any advice? Any good books to read?
Part of me thinks that this is just a long phase of him starting to become his own person, and someday he will eventually learn how to act appropriately in 'new' social situations. But then, if he doesn't, I feel like I will have lost all of this time when I could be helping him address his fears.
Do I continue to sign him up for classes to 'push' his limits and help him get more used to dealing with different social situations, or should I ease back and give him some time to socialize only in situations that he is comfortable in? I was really shy as a child (and still am somewhat today), but I would never have thrown a bucket at my swim teachers face, or refused to look at an adult when they asked me a question. So to me, it seems problematic, but maybe I am just worrying too much. My husband think I am crazy, but yet, he isn't the one who takes him to soccer or swimming or the park, so he doesn't see what is happening first hand.
I am toying with the idea of making an appointment with his doctor, but wonder if I am just an overly concerned crazy lady. The latter is probably at least somewhat true, but I just don't know how to deal with my child. I obviously want the best for him, and in order for him to reach his full potential he needs to be able to interact with the outside world. I am just not sure how to help him.