I suggest you take prostatitis into consideration. Long time's driving may cause prostate congestion which may further stimulate prostatitis. Prostatitis symptoms vary depending on the cause. They may include:
Pain or burning sensation when urinating (dysuria)
Difficulty urinating, such as dribbling or hesitant urination
Frequent urination, particularly at night (nocturia)
Urgent need to urinate
Pain in the abdomen, groin or lower back
Pain in the area between the scrotum and rectum (perineum)
Pain or discomfort of the penis or testicles
Painful orgasms (ejaculations)
Flu-like symptoms (with bacterial prostatitis)
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I get the same thing from time to time when sitting to long in my desk chair from doing school work. I get pain in the groin area. I think it has something to do with the circulation of lymph fluid in the lymph in that area for sitting for long periods of time. If you feel the area just to the inside of the hip bone you can check to see if your lymph are swollen. It makes sense that when you take a break from your long driving that your body is restored because the lymph fluid can move around again. You might look into buying some special seat for your truck that would be more ergonomic to increase the circulation to the legs. Otherwise I would recommend a new job that didn't require you to sit for so long at a time. If this continues I am concerned for you that it will lead to other health problems. Hope this helps.