Questions about her past.

This picture is in a frame on a table in our house. The other day Liv was looking at the pictures and I told her that was her when she was a little baby. She then asked me Who's hand that was in the picture. I said that it was" Anna Maria the lady who took care of her until mommy could come and get you". Liv got a little upset and said "no Mommy stay". Which is what she says when I have to leave her with someone even Vinay. Olivia is very attached to me, even still. I just held Olivia in my lap and said "no Olivia your with Mommy now and I will always be your Mommy". I didn't know what else to say but to reassure her that she will always be with me. We have always been open with Olivia that she was adopted, but how much does she understand now at 3? And how much should I try to explain to her?
Guess it's time to start reading my adoption books