I had anxiety since I was young and it went full blown when I was 13. It's a condition that involves both external and internal stimuli. This site has many posts on this condition, you could search and check them out. I also have a blog that document the causes, my thoughts, and techniques I learned to overcome anxiety. You can check it out at
Confilearn.com.
You should take your child to see a doctor, it will help. Depending on the severity of the condition, the doctor will prescribe medication accordingly. As someone who had gone through SA, one thing I've got to say is that it'll take effort and consistency to beat SA. Being supportive and understanding is the key to help your child.
Best wishes!