Very long shot here, but if it works, it's quick and easy.
It's possible your symptoms are caused by a lumbar spine condition.
Try McKenzie Press-ups. Sometimes there is instant (temporary and not total) relief.
You can do a self test. Do something painful. Try 3x10 press-ups. Repeat painful thing to see if any difference. If worse, there's a connection, and you just made it worse. Buy "Treat Your Own Back" book by me to find out what you should be doing. (or maybe find it free on the web). Of course you're going to Google/YouTube how to do McKenzie press-ups.
If you can't find anything painful, you could try your 400 tt again. Or a different functional test.
If it's better, hooray! If it's the same, you can still test the press-ups by doing 10 every 2 hours for 2 days, and sit up straight in between. Do the same if it's better (not just for 2 days) until no symptoms for 3-5 days.
Good luck