^Is it solely the outline of the vein, or is it like a bruise surrounding the general area of the vein?
Does it look like any of these photos?
If it looks sort of like the first 2 pics, it could just be normal bruising, which can happen when you miss, puncture through the vein, inject too fast, or otherwise damage the tissues. My one friend would have this happen every single time she injected, some people are just really sensitive. Bruises by themselves are not serious. A bruise is caused by distress to the tissue which causes capillaries or venules to break under the skin, allowing blood to escape into the surrounding tissue. As time progresses, the blood can seep further and spread. Gravity can draw the bruise downward from the injection site, which would explain why it appears as though it is following the vein downwards on the arm.
The last photo shows track marks due to scarring and likely vein collapse, but I'm not sure if it's possible for it to go very far down the vein if you have not injected lower in that vein. Mine only extend about a cm on either side of where I injected.
What you can do is apply a hot wet cloth to the area and watch it over the next few days to see if it begins to improve or not. It's best to not inject in this area in the meantime. Bruises can take a couple weeks to go away, but they should gradually fade and change colour. If it becomes increasingly painful, hot to the touch, or you have any other signs of infection seek medical attention.
