Sometimes with an assisted delivery the LO can end up with a preference for one side over the other due to a discomfort as a result of the assistance (sorry, didn't phrase that well, but hope you get the drift). Sometimes it's an injury (a small number of forceps deliveries get broken collar bones), but usually it's a tightness, either from the intervention itself or from how they'd got lodged in the birth canal.
It might not be up your street (and wasn't mine until my friend raved about it), but it might be worth looking into some cranial osteopathy from someone experienced with dealing with infants. I've no real idea how it works, but they somehow manage to release some of these pressures from the neck, skull, etc and people often see great results with feeding and wind related issues.