I get a shooting pain in my lower back - "the tailbone" when I sit for more than 5 min, since early twenties. If stand it relieves a bit. Any advice?

Coccyx. If the pain occurs with sitting in the tailbone, there may ba an issue with the coccyx. There are different causes of pain towards the tailbone region, including issues of the low back and sacro-iliac joint. I would recommend an orthopaedic evaluation to diagnose and give treatment recommendations.