What Causes A Lump On The Thigh?

My mother had a mass develop on the side of her left thigh, just in the past 3 weeks.........it has grown so rapidly that she cannot fit into several pairs of her pants. She saw a doctor and he has ordered some testing (an MRI) I believe. After he examined her, she said the area became bruised and was very painful. She has a history of blood clots. Could this be a clot? I would think this is not a usual place for a clot to develop. We also have many family members who have died of cancer (colon, lung).............very worried. She take NO medication, just vitamins. Other than vertigo, she is a very healthy 76 year old...........