l לבד is an adverb, not an adjective. It doesn't have feminine form, just לבד. However, as many other adverbs, pronominal suffixes can be added to it. If I understand correctly, it is rather literal and not used in daily conversation, and then it means alone, in the sense "on one's own". So, לבדי would be on my own, לבדה on her own, לבדנו on our own and so on.