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This sounds like venous stasis dermatitis. This is often caused by venous insufficiency.
This can be a precursor to more serious problems such as venous ulcers. Once these occur, they are very difficult to heal.
If your doctors have advised you that it was nothing to worry about, then perhaps they have evaluated you and found no evidence of the conditions that typically lead to this problem.
Venous insufficiency is a condition in which the flow of blood through the veins is impaired. The valves in the veins that prevent the back flow of blood become impaired or damaged. Thus the blood leaks out into the surrounding tissue - resulting in the discoloration in the skin that you see.
Being over weight causes stress and pressure on the veins and is an underlying cause in many cases of stasis dermatitis. Other common causes of venous insufficiency (leading to stasis dermatitis) are previous cases of blood clots and varicose veins, hypertension, and genetics. Weakness of the leg muscles that help squeeze blood forward may also contribute to venous insufficiency.
By genetics, we mean that some people have a propensity to weak valves in the veins.
Treatment depends on the cause. If you have not been evaluated for hypertension, this would be suggested. If you are over weight, then a change to a healthier diet and increased exercise will help prevent advancement of this problem.
Smoking is harmful on the circulation, so if you smoke, stopping will help.
It is important to protect your legs from injury (as they may not heal as quickly).
Compression stockings are usually recommended. There are brands now, usually available in drug stores and pharmacies that appear just like regular trouser socks (usually black in color though) but that go at least to the knees and provide compression and support for the veins of the lower legs.
Elevation of your legs when sitting may also be helpful, and it is best not to stand or sit in one place or position for very long; get up and move frequently.
Please reply if you have additional questions or need clarification so that I may address them until you are satisfied with your answer.