I feel an air bubble in my left boob. Is that possible?

It sounds like you are feeling the edge of the implant.  If the pocket is changing shape due to capsular contracture, you could be feeling an implant fold.

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.

 Which are probably feeling is the shell of the implant giving way as you generate pressure. Subcutaneous air can be present for a few days after an operation but never after that.

It is unlikely that someone can feel an air bubble.  It is possible to feel the edge or a "bubble" of the implant itself.  It can seem to pop in and out in an area of thin skin.   A personal exam by a plastic surgeon should be able to give you a more accurate answer.  Good luck. 

When saline implants are used for breast augmentation the implant is filled at the time of surgery. Normally care is taken to evacuate all of the air in the implant at the time of surgery. For several days after the operation there is normally some air and fluid in the pocket surrounding your implant. The combination of the air and fluid may create a "sloshing" sound or feel. After a few days the air and fluid is absorbed and this phenomena goes away. If you are noticing something unusual 2 years postoperatively and is very likely not an "air bubble". It is possible that you have develop some thinning of the overlying tissues and you are actually feeling the edge of the implant. It would be best for you to be examined by your surgeon to help make the proper diagnosis. Good luck.