Anxiety
Everybody feels anxious from time to time. It's a normal response to stress. Sometimes, being anxious can even improve your performance because you give more energy and attention to what you are doing. But some people find their anxiety starts to interfere with everyday things like going to work or mixing with people.
People with high levels of anxiety can experience panic attacks - sudden severe anxiety episodes that happen without warning and with no apparent trigger. Untreated, anxiety can lead to depression.