What Causes Twitching Sensation Below Armpit?

Every other night when I lay down on my left side I feel a twitching/pulsing sansation just below my armpit and next to my left breast. It does not hurt at all but I ve noticed when I take a deep breath I could barely feel it, but when I m letting my breath out is when I feel it the most, is only when I m laying down on that side is that I feel it. When I touch the area I could also feel that pulse like sansation there, is never concerned me before, but since now is happening more frequently is starting to worry me a bit.