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.