You don't say whether your little girl is very imaginative.
Whether this is true or not, it could be just a stage she's going through. As children grow more aware of the world around them, they begin to wonder about things, and that can sometimes translate into fears. They don't know what else to do with the things they're thinking.
If you were to ask her, casually, "What bothers you about going high in the swing?" what would she say? Try asking her. If she's afraid of falling off, you can say, "Keep holding on and you'll stay on the swing. You have a good grip there." That lets her know she has some control. If she says, "I'm scared that I'll go all the way around over the top of the bar and fall off," you can answer, in a friendly, matter-of-fact way, "Oh, I don't think that will happen. It's a l-o-n-g way up there, and I can't push you that hard. I think you're safe with me." If you're interested but not concerned, it may help her to be interested but not concerned.
When my children were little, they all - at one time or another - were afraid of the noise at the monkey house in the zoo. And I couldn't blame them. About all I could say was, "They ARE noisy! Let's go outside. What do you think they're saying to each other so loudly? Are they mad or just being silly?"
The dark can usually be conquered by a gift of a little flashlight all her own. In fact, I've known kids who declared themselves to be afraid of the dark for a while just to get a little flashlight.
You'll probably catch on as to when this behavior is a real concern (for that moment) and when it's a game. When it's a real concern, asking questions might help. When it's a game, you can choose whether to play it or whether to divert her attention to something else.