There are many reasons you could have a rash on your leg. For example, you could have a fungus on the skin on your leg...yes, you can get a fungus on your leg. Also, you may have an allergic reaction to a new product you're using, be allergic to a type of plant, or food. In addition, it could be venous stasis dermatitis from venous insufficiency...basically meaning you're having a circulation issues.
Your body carries oxygenated blood to your feet to keep them alive (so to speak) but it also needs to carry un-oxygenated blood back up from your legs and feet. If the veins get weak, it doesn't do a good job and your feet and legs can be pooling the blood there.
If you are concerned about a circular issue in your feet or legs, you should contact a podiatrist immediately!