My mothers lower half of her left leg is aching, and she doesn't know why or what it could mean.. any help? please..

Many things: Many things can cause leg pain including back problems, leg arthritis, circulation problems, vein problems, increased activity, trauma. She should see her primary doctor first, since that person knows of any ongoing medical issues that could be a cause. She or he can also examine your mother for some of these & other problems.

Many possibilities: The causes of leg pain could vary depending on the events preceding the onset of the pain. Common causes could be bony, muscular or related to her cardiovascular system (bloot clots). Other causes could include back/nerve root related issues or nutritional issues. Start by having her visit her primary care physician who will help narrow things down.