She was a few weeks early, and with the babies I've seen that are small and early, it seems to take them a little while to catch up. Then they all-of-a-sudden wake up and gain/grow normally.
Is she staying awake for her nursing sessions? That's probably where I'd concentrate most...keeping her awake and alert for those 10-20 minute sessions at least every 2-3 hours and letting her sleep/grow the rest of the time. Undress her and wet her feet/hands/face if it helps to wake her up a bit.
Pumping and topping her off with EBM is a very good idea. Little ones tend to get tired and fall asleep before they finish feeding...and nursing is much harder work for them than bottle feeding.
She'll be fine...just keep feeding her. Before you know it she won't be so quiet anymore.