Hi

Firstly, congrats on your first experience!
I find it so much easier to project whilst lying on my back, I seem to just go straight to sleep if I lie on my stomach. I often have exactly what you described - 'rolling' over and finding that your actual body hasn't move along with you! A very odd experience when you're not expecting it, lol.
Although I don't know much about the solar plexus, I know that whenever I experience 'astral pain' (like from a negative entity attack) I feel it in my solar plexus. Also, if I rush back to my body (when I've had a bad/scary experience) I get the pressure you were saying about in my solar plexus. The solar plexus chakra is in charge of the nervous system so I guess it's the most sensitive part of your astral body and so that's why everything feels as though its happening from there

- I'm afraid I don't know much about it.
It's really common to feel as if you're released through the solar plexus, I've often read about it - and it is one of the techniques to project, so I've read in numerous books anyway.
Hmm...I haven't really provided any useful advice here have I, lol

But i just thought I'd share my experience on the matter all the same

Pixie