A.,
Please stop thinking you might be to blame for this. While she might remember you yelling at him, I highly doubt it scarred her. And I don't think your break up did either.
Although that probably won't stop her from trying to use the break up as an excuse (for anything) when she is older! ;-)
Instead of thinking of her as a loner, think of her as independent. If you are around when she freezes up when asked questions, kneel down beside her and say something like "Kalie sometimes doesn't answer when someone asks her questions. I don't know if it is because she is feeling shy or scared or something different. But at our house every feeling is ok, and when she is ready she will answer."
You need to get the teachers on board with this, too. They can give her the option of nodding, or shaking her head, but I would say there needs to be some response that she gives. Maybe a thumbs-up for "I am ok". Then, mostly at school, work up to giving a verbal response. Even if it is "I don't feel like talking right now".
She gets to feel whatever she feels, although she will eventually have to start responding. Maybe instead of "What is this shape or color" they can say "Kalie, point to the square" or "Please point to the red flower." Then she doesn't have to answer.
And you should probably quit asking about her time with daddy - you know she probably didn't see him much, and just say "I hope you had a good time this weekend".
Kids have accidents randomly. Please don't worry. She is so lucky to have a mom who loves her soo much. I know how hard it is to be a single parent, probably even harder when you are so young, so give yourself some credit for everything you do!
K.