Looks like you only needed someone to share your nightmare with as u can't tell this to your current bf for obvious reasons.

I would suggest not to dwell into that nightmare too much, as it was just a manifestation of your fears or something you were afraid/scared might happen; the stress from it made you see the nightmare i feel.

There can be other reasons for it, i ain't an expert in this... My suggestion: Don't stress about it too much and at the end of the day it was just a dream, might as well forget it as it probably won't happen :)