step mom advice.....?

My boyfriend has a daughter who's 5 and just the cutest girl ever. 
We're getting along pretty well with her but when I slept over last night (she loves co-sleeping with him when she stays at his place because he's travelling a lot for work) she was like "oh" and was pretty sad. 
He said we could all sleep together because he and I share a bed.
She was like "are you my new mommy now" and I think she was maybe even sad. 
I told her it's okay and she can sleep with her daddy and I'll sleep in the guest bedroom she still seemed sad though.
My boyfriend didn't want me to at first because he obviously wants her to like me too but I insisted on staying in another room so she wont feel bad.
Today he said he told her we're together and we love each other and stuff and said it's normal and she really likes me too and she might just be a little jealous but I feel so bad.
We used to nap with her and got along SO GREAT honestly it was always us against her dad but once she found out we're together she was sad and I don't ever want to get between her and her dad.