Eye rolling while feeding

The last couple of weeks LO has been rolling (not jerky movement slowly lowing eyelids slightly and rolling eyes) his eyes toward the back of his head while he is eating.  The first time he did it I thought it was cute.  The best way to describe what he is doing is that it looked like he was lowering his eye lids and rolling his eyes back like he was completely content and in ecstacy that he was eating.  Originally I thought it was a one time thing like he was so hunger and just so happy that he was eating but now he does it once or twice each time he is eating. 

 Should I be concerned?  Anyone else's LO do this?