Amazing advice here already.
I actually started to get the same problem, I have always been fit and healthy but since working on my site all day every day I have noticed my posture has changed and I get back ache when sitting at the computer. This happened in only 2 weeks! Unbelievable!
I started to exercise every day again (after taking a 2 week break from exercise) and stretched off my hamstrings by lying on the floor and pulling my leg towards me with a band attached to my foot, this helped dramatically! But by far the best thing was to do a good run every day and stretch off afterwards. Now my back still feels uncomfortable, but I am doing core strengthening exercises to try to make my abs stronger again. Also get a good computer chair, that is really important. It really is amazing how the human body can just put itself out of shape in just a few weeks!