Recurring sharp pain building up to last for about a minute under my left cheek bum. is this a disc problem or a thigh problem, or more serious?

Yes and no: Likely from the pyriformis muscle which can be from strain or from the sciatic nerve which frequently runs through the middle of the muscle. If you are driving distances, keep your foot on the same side as the buttocks pain (likely left), in the 12 o'clock position and see if the pain improves. Check with an osteopathic physician for a physical diagnosis and to suggest physical therapy if need.