Flashback?

  Something interesting happened this weekend.  When Sky woke up from her nap on Monday, I brought her into our room where D and I were reading in bed.  We were tickling her and playing "Where's Sky?" with the sheets and she was laughing and having fun.  She started snuggling into me after a little while and D started singing "Go To Sleep, Little Baby", and she burst into tears!  She just started crying like her heart was breaking and didn't stop until I picked her up and started rocking her on my shoulder.  D and I just looked at each other, both thinking "What was that?!".
  The song, "Go To Sleep, Little Baby" was one that D used to sing to Sky a lot when she first came to us but hasn't been singing much recently.  He thinks she might have had a flashback, that the song might have reminded her of how she felt when he sang it before, when she was missing her aunt and I think he's right.  D stopped singing the song as soon as Sky started crying and I don't know if he should try it again sometime or just remove it from the song list.  If he sings it and she crys again, is it helping her to work through the sadness, or should we just leave it be?  Thoughts?