It could be anxiety. Relaxation techniques may help. Try to take deep breaths where you breathe in through your 'belly.' Think of a sleeping baby. Can you see its stomach rising and falling? This is considered the relaxation response. When you breathe with your upper chest, that induces the 'fight or flight' response, which perpetuates anxiety. Also, clear your mind, sit back in a relaxing position, play soothing music if you have it, and breathe.
If this doesn't help, you may have a hormonal imbalance. If you have difficulty breathing, chest pains, and tachycardia, it could be hyperthyroidism. If your heart is beating over 140 bmp or skipping beats, you may need to go to the emergency room. Are you thin? Eat a lot and can't gain weight? Those are signs of hyperthyroidism.
But if it calms on its own, it may be anxiety. A psychiatrist visit will confirm this. I wouldn't go to a GP because they cannot treat you (they can give you meds, but will refer you to a psych--might as well save some money and go straight to the specialist).
Let us know what you find out.