I think you have a very ACUTE back issue, not the kind many of us with chronic issues have. You have the kind of problem where suddenly, doing some minor action (I once got his, decades ago, picking up my purse), it feels like a huge spasm is created.

The recommendation for that is generally rest. I think most doctors would treat your pain aggressively, if only to knock you out cold and keep you from moving for a few days. They'd give muscle relaxants and pain pills as much for the sedation as for a real effect. Then, you lie in bed, or the floor, and alternate cold and heat as seems to work best for you. With rest, these things usually mend in about a week.

But over and over I've heard that one should rest acute spasm.

Oh, and sitting is way worse than any other position--

You might check other opinions, but I think a few days on your back might really be good for you.