Congrats on your OBE!
Pain during these experiences is not unusual. I too have felt intense pain before - but in the instances I've experienced it it's been more localised to certain areas (example hands).
Yes, for me too it extends to the next day.
A while back I attempted the exercises in 'Mastering Astral Projection' by Robert Bruce, and specifically the energy exercises drew me to conclude that this pain during OBE's could actually be small energy blockages.
Some experts suggest gently sweeping your body's energy up and down to loosen it before sleep (or astral projection attempts).
This article here talks about the vibrational state, but also has some information on the energy body and how to gently work with it when preparing for astral projection:
I hope that helps, and best wishes for more cool experiences!