What causes lower back pain when sitting?
Lower back pain that shows up when you sit is a common complaint among readers in Garfield, and a desk chair is only one of the places it appears.
Sitting and rest breaks
One instruction about sitting is worth knowing. OrthoInfo (AAOS), in its guidance on a herniated disk in the lower back, says that when you resume activity you should take rest breaks throughout the day but avoid sitting for long periods of time.
Reading that advice carefully
That guidance sits on a page about one specific problem, a herniated disk, so it does not tell you that a disk is what hurts you. It does show that long stretches of sitting are something clinicians ask people to break up.

A useful habit is to note when the pain starts, how long you had been sitting, and what eases it. That record helps a clinician sort out the cause far better than a guess from a web page.
Where to look next
Start with general references: OrthoInfo on low back pain, NIAMS on back pain symptoms, types and causes and MedlinePlus on pain. If pain keeps returning, Cleveland Clinic's overview of pain management explains the kinds of care that exist.
Before any visit, ask the office what the exam fee covers and what a firm quote for any further testing would be.
Sitting is where you notice it
Separate what is known from what is assumed. Sitting is where you notice the pain, not proof of why it is there. Pricing notes like these are not medical advice, and new weakness, numbness or severe pain is a reason to seek care promptly.