Warm it up. No sudden moves, it may get worse, like you "scare" it and a chain reaction will start. After warming, find a corner of wall and lean on that muscle. Scrub it from spine away like a bear a tree. Or just lean on it. Don't hurry. Then get someone to rub some pain cream or at least some alcohol into it. Point is the muscle is shortened, might pull your spine or cause reaction in other muscles. You need to relax it. Warmth helps.
Discussion
After you feel it has slightly loosened and relaxed, like after hour, try to stretch it SLOWLY. Again, don't scare it. And go back very slowly, lean with your hands on something so it doesn't panicking and shorten again. Sleep with back in warm and avoid sudden moves. Don't go into a hot tub, you might not get out.😃 Sincerely, a masseuse. Oh yeah finding one helps.