Testicles hurt after wearing tight underwear while driving ~8 hours a day for a couple of days. it's been more than 3 weeks...what could it be?

Here are some ...: Wearing tight underwear just provides the snugness to hold testes together and has been a useful way to help ease testicular pain from various reasons, especially for postoperative care after scrotal surgery. If indeed such wearing has bothered you, you would need to see Doc, preferably a urologist for detailed Hx, physicals, & tests as needed so to sort out what might have been happening to you.

Varicocele: The sitting position causes a pool of blood in the veins of the pelvis. Drivers are prone to Hemorrhoids when the problem is in the rectal veins or varicocele if the pool is in the pampiniform plexus that drain blood from the testicles. A doppler US will give you a more definitive diagnosis.