Why Would One Experience The Feeling Of A Lump In The Throat While Swallowing?

17 year old daughter is experiencing the feeling of a lump in her throat. When she swallows she feels like there is a lump. She has been tested recently for strep and mono - both were negative. She previously described it as a sore throat but says it is more like there is something in the back of her throat when she swallows.