Like rubyted suggested, it's never a bad idea to check in with a healthcare provider when we're unsure about some part of our body. With what you're describing though, Ann, you're probably happy to hear that it's much much more likely to be your body behaving as it's designed to behave, and not any medical problem. There's very probably nothing wrong with you at all
"How can my body be designed to behave this way?" you might ask. Well, when a vagina is just existing there in your body being a vagina and not doing anything in particular - so, childbirth isn't happening, and you're not feeling aroused and wanting insertive sex - the walls are naturally closed together. The vagina isn't a one-size tube into your body: it changes size a
lot depending on what you do with it and how you feel about what you're doing. When you're not really doing anything with it, the walls are going to be closed together, and that's probably what you were feeling when it felt like the walls were shut tight.
The vulva (your external genital area) and the vaginal entrance are quite delicate and sensitive. Often, touching them or trying to put something in your vagina when you're not aroused will feel uncomfortable or painful. If you're feeling tense, that tension in your body usually makes the vaginal entrance close up, and makes it difficult and maybe painful to put anything inside while you're tense. Too, friction can be a problem: it's difficult for many people to put a finger or tampon in the vagina when it's dry. Using some extra lubricant makes things more slippery, easier, and a lot more comfortable.
As you said you haven't really had sex ed, here's our detailed piece on body parts:
Innies & Outies: The Vagina, Clitoris, Uterus and More
and here's an advice column that answers a question a little different from yours, but where the answer describes how the vagina changes when you're doing different things with your body and mind:
http://www.scarleteen.com/article/advic ... e_for_real
and an advice column where the answer talks about how to slowly get to know your body, without pain and without worry:
http://www.scarleteen.com/article/advic ... hould_i_do
Take your time with those if you need, there's a lot of information there!
Do you have any questions about what you read or about your own body?