It could be one of two things. First, maybe there wasn’t anything held in that space, and so this exercise was just deeply relaxing; or two, there is a LOT held in that space, and our body chose to check out rather than engage with it.

Since you normally have difficulty sleeping, and the sense was almost like passing out, this strongly suggests the latter, so I suggest revisiting the exercise and only doing a little bit — if you start to get that passing out feeling, STOP, orient, and see what emerges.