What Causes A Massively Swollen Gland In The Neck?

my daughter is 4 years old and had a swollen gland on one side of her neck it was massive my parents took her to the doctor and told her they couldnt see infection but give her an antibiotic. I took her that nite to out of hours doctor and he said she had a swollen tonsil. Her antibiotic is finished this friday and it has just started to go down but im so worried bout it.