Anxiety starts as a simple thought. It then grows into more thoughts until it's all your are thinking about. Next, it stirs negative emotions within you and then you take action. Here is an example to help you better understand the cycle.
I'm sitting at my desk in school and the teacher is lecturing on some topic. A thought of how it's quiet and there's a test coming up. (Thoughts) I don't like silence. What if people can hear whatever noise I'm making? Why is everyone looking at me, even though their backs are turned? My stomach hurts. I don't feel good. I'm ready to get out of this room. (Emotions) The intensity rises and soon, I can't stand it any longer. I have tried and I can't do it any more. I get up and bolt out of the room. (Actions)
That's a sucky, but hopefully, good enough example. If you have a better example, please share!