Why did my Breast deflat?

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Hi I’m a first time mom! My baby is 12 days old I have been mostly breastfeeding. My breast grew so much during pregnancy and again once my milk came in. But now they are so deflated they never feel engorged or full, i dont pump often I do feed my baby often tho. But they literally are on the floor no fat in them just saggy skin I’m so disappointed and confused. What can I do to get some fullness back I’m only 21. Why don’t they get full anymore?