Is the fussiness only after feedings? If so, as Leighanne suggested, I'd look at your diet. Does she have any of the other signs of a sensitivity to something in your milk? In general, a nursing infant who is sensitive to something in the mother's diet will be fussy and gassy, particularly right after feedings, but will also have several of the following symptoms: greenish, frothy or watery, foul-smelling stools, severe diaper rash or redness/"burning" around the anus, infant acne, particularly spreading from the face to the body, cradle cap or flaky scalp, dry/static-y hair, frequent hiccups, frequent spitup, possible reflux (you can sometimes tell if a baby is having acid reflux by their arching back and leaning their neck back), and trouble sleeping.
Otherwise it could just be one of those phases
