What does it mean when you are so nervous that you begin to hear your heart beat inside your head?

Theresa is correct. You reached a state of anxiety that started to affect your bodily systems in a way that you could be aware of it. When this happens again, inwardly and gently acknowledge that you are upset and immediately put your attention on your breath. By doing this, you begin to focus your mind on something else. The process of watching your breath becomes very soothing for your body, your mind and your emotions. It may take a little awhile but stick with it. It helps if you can close your eyes too. That way you do not get distracted by your surroundings. Gently watch your breath going in and then going out. No need to control it in any way. Your breath will start to slow down naturally and easily. Then you can scan your body to see where you are holding tension and relax each part by intending first, then releasing the hold all the while breathing into that spot.
This process will give you a sense of control and you can do this anywhere, anytime without anyone noticing. If you have your eyes closed and someone asks what you are doing, just tell them you are meditating. They will be impressed that you are doing this for yourself. If they give you a slide remark, tell them they might be right and keep focusing on your breath. No one can stop you from thinking or feeling inside. You make the choice that is healthiest for you. You will win everytime.

panic attact

Ask_Sean Says, Exactly what Anahata said :D