Anxiety is a a general feeling of unease and tension in the body which happens when your body feels the need to prepare for a stressful situation. E.g when you are going to do a cartwheel for the first time. Your body does not know how to prepare. The heart rate increases to allow yourself to be ready to deal with anything that might happen.
You may feel wanting to stop still and just look aroud thinking what is going on, after this you will start thinking of what is happening and what might happen depending on what you decide to do. Your body and mind in this moment is protecting itself from harm. It feels doing nothing will be the best idea for the stressful situation to pass.
What we said in the first paragragh the body is still making the heart beat faster. When you do move, you will be able to do so without difficulty. It would be like if you pressed the accelerator and brake on a car at the same time, your body would be trying to get faster and faster, but unable to go anywhere. Being stuck on the spot and your body working very hard is fine to do now and again. Always feeling like this it can be very tiring and start to damage your body.