I suspect you might have some tension in the muscles of the lower back, particularly since you've not mentioned any obvious buttock strain.
Because the pain travels down your leg, it does suggest that some inflammation or wear to the lower back is present, as it would often be associated with compromised circulation to the lower limbs, or a nerve has been 'pinched' in the lumbar area, resulting in sciatica-like pain.
Bad backs are equally likely to be due to strains, caused by lifting heavy weights, as they are inactivity such as sitting for long periods in one position in the office or in a car.
When you feel able, try the following. I've personally found them excellent for preventing recurrences (and later have discovered they are exercises used by physios for bad backs):
- On your hands and knees, arch up your back, aiming to rock slightly back and forwards. This helps stretch the upper and lower back after periods of inactivity (maybe do it when you get in from work?).
- Staying on your hands and knees, raise your left arm and right leg so that they are straight, then bring them both underneath you so the elbow and knee almost touch. Repeat this for 30 seconds, rest, then do the same with the right arm and left leg.
- Lying on your back with your legs bent and feet on the floor, raise one bent leg so the knee approaches your hands, then grab behind the knees and pull towards your torso. Repeat on the other side and then try with both legs.
- Finally, the best thing I ever did for a bad back was sit-ups. Always start with bent legs and tuck your toes under the edge of a chair to provide a brace and lift the shoulders only (it isolates the abdominal muscles more). When you get good, lie on you back, then lift both your shoulders and straightened legs off the floor, balancing on your bottom, hands on your ears. Then pull both the legs in and torso up so the knees meet with the elbows. Don't pull the head with your hands as you might strain your neck.
The abdominal muscles oppose those of the lower back and when the latter get stronger, it causes the lower back to arch in an excessively concave fashion.
Stronger abdominals will help pull that part of the torso straighter again.
When you are done, try finishing with some 'Jane Fonda' style twists which means standing and looking behind you whilst twisting from the torso, alternating sides.
You can find more information and a video with some of these exercises on my lower back pain page.
For now, though, do try some Atrogel® 2-4 times a day, some Atrosan® Devils Claw if you have experienced the pain for a number of weeks or months and those stretching and strengthening exercises.