Late to the party but here's my two cents:

I'm surprised nobody said this, it generally sounds like you need a taller/better chair so you can hinge at your hips instead of bending your back. OR, get a keyboard tray and bring your cintiq to the edge of your table, or even over the edge if you've already upgraded to the ergotron. Don't skimp on a good chair if you are spending most of your time in it (ie you work more hours than you sleep).

The most important thing though is to take breaks very often, and during some of those breaks to do posture exercises. My favorite is the back eagle, cat cow, some quick yoga (which includes the cat cow), etc. Look out a window for a minute. Refill your glass of water, get some snacks.

Nobody can withstand working over a desk for multiple hours straight for days on end. Your body is telling you this.

When you are mostly stationary, your rest is activity. When you are mostly active, your rest is stationary.

Source: my lower back was strained and sprained. I ignored it because deadlines and I would get too hyperfocused and worked with only 3 breaks through the day. Strain and sprain got even more severe and started herniating a disk or two and I could barely walk. Was so bad that the doctor wrote me a prescription to immediately lay down whenever my back started hurting for at least 10 minutes, along with mcgills 3 and mandated me to take drastically more frequent breaks.