If you find it increasingly difficult to sit without pain in your rear end, you might have hemorrhoids, a condition caused by swollen veins in the anus. Sometimes the condition exists just inside the anus and the veins aren't visible, while other times the swollen veins are external.
The causes of this vein-swelling could include pregnancy, straining during bowel movements or too much sitting. However, the condition could also be a sign of a bigger problem, such as liver cirrhosis or anal infection. Discovering such a cause sooner rather than later would certainly be worth the mild embarrassment of bringing up this derriere pain with your doctor.
However, even if your diagnosis just amounts to "garden-variety" hemorrhoids, getting a doctor's advice on how best to treat the condition will also be worth the trouble. Hemorrhoids can cause blood in the feces, painful bowel movements and itching, not to mention an unpleasant experience every time you sit down to work, drive or relax.
Not seeking treatment would likely worsen the situation, perhaps even necessitating otherwise-avoidable surgery. If you seek a doctor's advice early on, though, chances are you may only need to use over-the-counter topical creams with corticosteroid or lidocaine, and other noninvasive remedies, such as heat treatment, aka infrared coagulation.