Last time I saw her before I became Guild master was at the Twilight Sepulcher talking to Gallus. Then the next time I found her was when my character became the Guild Master, but I can never find her again if I check both areas and I never found her. Is it a bug or is she wandering around as a thief?

she should be at the thieves guild i think

She tells you she will be living in the Nightingale hall from now on and hopes to see you there from time to time.

She roams around Skyrim looking for lucrative opportunities. Every now and then you may encounter her

I found her in Snow Veil Sanctum. Gives me a line about sharing the wealth and all I see are dead deugr. This was after going to the other two Nightingale spots.