Is it common to have an air bubble in stomach after BA?

This Is perfectly normal and will disappear in the next few weeks. It is from air and fluid (irrigation, local anesthetic, serum, etc) in the pocket which will be naturally absorbed over time.

It appears that what you are describing is subcutaneous emphysema. During a breast augmentation surgery, a pocket is created. Air will get trapped in the pocket after the implant is placed. This air can travel under your skin and move into your abdomen, back or neck area. This condition usually resolves withing 3-5 days.If it persists beyond this period, you should see your surgeon.

Air bubbles called crepitus can occur in the breast skin and upper abdominal skin  in the first days after breast augmentation.Of course you want to check in with your plastic surgeon to be certain.However as long as you not having any breathing difficulty this most likely is crepitus but again check with your doctor.