Feeling the Need to Take A Deep Breath After Developing Concentration
If your breath becomes shallower faster than the extent you have calmed down the fabrications this tend to happen. Sometimes the breathing stops and suddenly after someone time you gasp to breathe. But again these inconsistencies reduce after some practice so might be safe not to pay attention to it and keep your focus on the breathing process. 

Alternatively you might be trying to control the breath even without your knowledge. You might be "forcing" the breath to be shallow causing a gasp to breathe.