Hi LP,
I would suggest there are three reasons (at least) to suspect it's not an SF.
1) SF pain is typically very focal - you can literally point with a finger to the site of the pain - this is especially true of a tibial SF which is about the most common type.
2) SF pain almost always stops when you stop running, though there may be local tenderness or even bad pain, when pressure is applied over the point of injury.
3) SF pain does not typically ease off as you run further - it will either stay constant or get worse.
NIckL is absolutely right to suggest than an x-ray will be of little help - it is highly unusual for a stress fracture to appear on x-ray, and usually when they do it is during the healing phase when new bone is being laid down. MRI's have a much better record on this level but your GP will be reluctant to do this as they are expensive and your symptoms are not classic SF by any stretch.
There are a few more things in the lower leg than muscle and bone (thank god - or you wouldn't be able to walk, let alone run) - there are ligaments which attach bone to bone, tendons which attach muscle to bone (like your achilles) and there are stacks of nerves, blood vessels, - oh and you've got specialised facia that compartmentalize various muscle groups, particularly in your calf.
In your first post you said the pain starts in the lower area of your right shin and then gradually moves around the ouside of your calf and up to to your knee (presumably during a run) . In a following post you said it was all over the front and side of your lower leg (presumably after a run). You also say it's not sore all the time (presumably at rest) but that the pain comes and goes.
Some questions ...
Other than the lower front shin, which side of your leg is the pain on - outside or inside or is it actually in the calf at the back ?
How spread out is the pain ?
Try applying pressure working all the way along the shin muscles from the bottom near your foot to the top just under your knee checking both the soft tissue on the outer side of your shin and the inner side of your shin - identify which areas are most tender and the extent of this. Is there tenderness in your calf or the back of your knee ? Nip your achilles firmly but not too hard between your thumb and forefinger along it's length - any tenderness ?
Does the pain only move about during a run ?
Does it come on when walking ? when sitting down ? when lying down ? one, two or all of them ?
Is it a dull ache, does it throb, do you get any tingling or numbness ?
Did anything change prior to the onset of the pain ? (e.g. new trainers, increased mileage, started speed work, new running socks, shoe inserts )
Any help with these questions might throw further light on your issue.
Regards, Groz