Hi! Thank you so much for your helpful reply!
I rebooted my Mac. (I also used CleanMyMac, and First Aid in Disk Utility).
I quit all my Applications. I even un-installed many Apps (via CleanMyMac), including Pages app.
Though I did not know about Press + Hold Shift-key when launching Pages (I'll keep that in mind next time - thank you).
After all of the above, it did not resolve the spinning wheel - BUT I noticed it was THAT particular Pages document, not the others. So, I just duplicated that document, and I was able to work on the new copy without any spinning wheel and without changing anything on the document. Weird! Maybe there was some disruptive formatting or images that were causing trouble on that particular Pages doc.
Your message was super helpful - to know for next time if it happens.
Cheers,