Air bubbles don't normally exist under the skin in the neck, or anywhere else in the body. Although it may seem as though this trapped air must have come from the middle ear, this would be a bit unusual, since you have no ear pain or other symptoms, such as a ringing or roaring sound, or even loss of hearing. Middle ear barotrauma can easily damage the tympanic membrane or one of the more delicate internal membranes associated with sound transmission inside of the ear.
Although no one can say for sure, it is more likely that this trapped air - or subcutaneous emphysema - originated from pulmonary barotrauma. Sometimes the lungs are over-pressurized, for instance, when we have difficulty clearing. This can happen when we attempt to put air into the sinuses and middle ear with more force than is necessary. This can actually increase the amount of pressure in the air spaces of the lung, which then allows air to escape through lung tissue, where it can travel up to the shoulder, neck or even the face.
Subcutaneous emphysema does not require recompression treatment. However, it is a good idea to be evaluated by your physician to determine the underlying cause of the air bubbles.