I don't know what causes bubbles in breastmilk but the same thing
happened to me with my second child.  There were bubbles in the milk no
matter which pump I used and even with hand expression.  It seemed as
though it came out of the nipples in bubbles.  Is that possible?  It
happened before it hit the bottom of the container, in fact, they were
visible immediately upon expression.  My gut reaction was that it was a
sort of "gas".  I had a colicky baby at the time and I wondered if the
two weren't somehow related.  I didn't need to express very often and
didn't notice how long it lasted.  The baby stopped being fussy at about
3.5 months.

Dee Thompson
LLL of Beaufort, SC