Hi,

It probably is the result of slightly less than optimal attachment - he's having to excert more suction than normal to keep the breast in place, on the other hand, nothing to be too alarmed about. Make sure he is turned into your body, chin to chin, chest to chest, you wait til he has a wide open gape and you hold him close. IMO, you should just be able to see the curve of his heek against the curve of your breast. If you can see his lips, move him in closer. Sometimes you just about turn yourself inside out trying for that "perfect" attachment, when nothing but time will sort it out, so don't be alarmed - it will get better.
Barb