This sometimes happens if your breastmilk is high in lipase, the enzyme that breaks down fat in the milk.  Depending upon the levels of lipase, you may also notice this smell after you have cooled your milk in the fridge.  It is still safe for your baby, but if you are concerned, you can scald your milk to deactivate the lipase before storing.  Heat the milk until small bubbles form around the edge of the pan (but not to boiling point) and then quickly cool and freeze it.  However, once your milk has been frozen and acquired this smell, scalding will not help.