Hello there...considering the pattern of your pain ie after prolonged sitting and then not able to touch your toes..... it could be a classic example of mechanical disc problem and hamstring shortness. A disc is a structure which is present between the 2 vertebras and hamstring are a group of muscles at the back of the thigh...which helps in flexing the knees . Inflamed muscles are aggravated by sitting too long in one position.also if you have stressed your back recently or have gone on a long ride or bus trip could also trigger this phenomenon.  What you need to do--Get up every 30-40 minutes from desk, walk around a little, hold your hands at the small of your back & bend back slightly. The backward bending helps to relieve the pressure & compression on the spinal discs caused by sitting in a chair also do hamstring stretching exercises(which any certified gym trainer or a Qualified Physio can give you a demo) If your pains n symptoms dissapear with this well n good...or else resort back to me for some detail discussion Regards Take care.

Pain after prolonged sitting to standing indicates mechanical disc pathology. It is common who do work in poor sitting posture. If you get pain when you wake up from the bed in the morning, that is confirmation for disc problem. As long as pain not radiating down to the leg, it is manageable with simple exs and with posture awareness and correction.