Marie,
It sounds like the baby is not getting enough of all that great, calorie rich
hind milk.  Is she pumping?  Is she nursing often? Does baby respond to any
of the usual stay awake techniques? My second was 36 weeks, 5lbs 5oz at birth
and very sleepy (no meds during birth).  At 3 days she was 4lbs 15 oz. but
with more than adequate urine and stool production.  By day 5 she began to
get it together and nurse longer.  The visiting LC suggested I nurse one side
and pump the other alternating sides at each feeding.  This seemed to work.
 She quickly began to gain weight and was a real "rolly polly" by 2 months.
 Maybe this could help her as well.
Diane Karnbach, LLL Leader