No obvious stressor is present, but something trips your body's alarm system. If panic attacks are frequent, or if fear of having them affects your activities, you may have a condition called panic disorder. This condition can become debilitating and destructive. Consider learning techniques that can help you relax. There you conjure up an image you experience with your senses to help you alleviate physical symptoms.
Relaxed breathing is deep and slow.
You can slow down by practicing deep breathing. Inhale slowly to a count of 4, then exhale slowly to a count of 4. If you have symptoms of a panic attack, seek medical care. Knowing what you experienced is the first step in learning to manage it. Your doctor may give you information and teach you coping techniques.