:( Swollen Gland

My 5 y/o chihuahua has a swollen gland one the side of her neck.  However, she appears unaffected by this and has been eating and playing and being her usual cheerful self.  What can be causing this? She has not eaten anything out of the ordinary, nor has she undergone any rrecent medical procedures.  I did schedule an appointment with her vet for a check up, but I am still worried since I have heard so many stories of purebreds who are more prone to medical issues and have a shorter life expectancy.