Your mother most likely has lower extremity edema which can be caused by cardiac disease, vascular disease, or an embolic process. Lower extremity edema is a result of insufficient venous return to the heart resulting in blood and interstitial fluid to pool in the legs. Treatment depends on the cause but usually involves leg elevation, diuretics, and inotrope/pressor drugs. Complications of chronic leg edema include statis dermatitis, ulceration, or even cellulitis.
Your mother’s acute worsening of leg swelling should be evaluated by a physician as it may indicate a worsening of her underlying medical condition especially if she is having increased shortness of breath (indicating fluid accumulating in her lungs as well). If her leg becomes cold, numb, or painful it could be a sign of a serious medical problem involving a clot in a major vessel and she should be brought to the emergency room without delay.