This rash could be a lot of different things, and it will require talking to your primary care doctor
or your dermatologist
in order to figure out what is going on. Some of the most common possibilities would be:
1. Venous stasis dermatitis. This is a darkening and inflammation of the skin in people who have chronic swelling in their legs. If you have a heart condition or another medical problem which leads to swelling, then this would be a possibility.
2. Varicose veins
. The skin over varicose veins often becomes irritated and itchy, leading to gradual thickening and darkening. This is especially possible if you have many large, dilated varicose veins.
3. Fungal infection. Skin infections caused by various fungi can present with itchy, scaling rashes that can be either lighter or darker than the surrounding skin. Fungal rashes are often circular with a lighter color in the center.
4. Contact dermatitis. This would be an inflammation of the skin due to a reaction to some chemical, beauty product, laundry detergent, or something similar.
As you can see the list is long! I agree with your concern about the rash spreading and suggest that you get in to see your primary care doctor as soon as possible.