Interesting that in this email you mention the reaction to lullabies and, without making the direct connection talk about many losses in your family. I believe your intuition has opened the connection for you.
Although often used as a sleep aid for infants the history of the lullaby has some dark overtones as this Wikipedia article can explain. Consider the one you mentioned, Brahms, part of the lyric goes (italics added):
Good evening, good night,
With roses covered,
With cloves adorned,
Slip under the covers.
Tomorrow morning, if God wills,
you will wake once again.
…and what about rock-a-bye-baby?
on the tree top.
When the wind blows
the cradle will rock.
When the bough breaks,
the cradle will fall.
And down will come Baby,
Cradle and all.
These are hardly comforting words provoking security. In fact, not soothing at all. The further possibility is that the lullaby was used by your mother or other caregivers when they were about to leave — typically a scary event for an infant to be aware of. The singing of the lullaby would come just before the loss of the caregiver. If the lyrics and the loss are connected the result would be devastating.
Of course, these are all speculations, but I can understand how lullabies could become a trigger as a conditioned stimulus to a separation fear or sense of abandonment. Hearing the lullaby would be emotionally linked with impending loss — and the emotional reaction triggered.
I’d treat this as a conditioned response and make an appointment to meet with a cognitive-behavioral therapist to help untangle this emotional reaction. There are many ways this can be done, and a good therapist will be able to walk you through the many ways this can happen.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral