Have a sudden swollen lump under chin. Could this be caused by ear infection, mono, or cold sore?

Lymph node. This may be a reactive lymph node. These are the lymphatic organs that make antibodies to foreign, non-self invaders in our bodies, which is often infection. In this area, the areas that drain into those lymph nodes generally would include the lips, skin around that area and the the oral cavity. So, it may be a reaction to a cold sore in the area. Get checked by your doctor, routinely.