Anne--I have seen a number of preemies who nurse short periods of time and
frequently break the seal.  If she seems to be getting enough milk and output
is fine, I would look into overactive MER.  I see quite a bit of it and when
you add pulling off + colicky baby I always think overactive MER.  I would
suggest that she see an LC to verify this as tx for this problem could be
disastrous if it is indeed NOT the problem.
Hope this helps!
Diane DiSandro, BA, IBCLC
Audubon, PA--LC in private practice