Hi my mom has a really sore knee the past few days. It is painful at the back of the knee and stiff at the front any idea what it could be?

Meniscus tear? The pain in the back of the knee is usually associated with increased fluid in the knee. Since this is acute and in absence of injury the most likely cause is a degenerative tear of the meniscus. In 33 percent of the patient this is treated with anti inflammatories and rest within 3 weeks, if not better, then she needs to be further evaluated by a doctor.
Patellar pain. It could be patellar pain, or kneecap related pain. Or, it could be arthritis. She should be seen by an orthopedic knee specialist for evaluation.