What Causes Swollen Lymph Nodes Around Neck?

I am 42 years old, female and i have a swollen tender lump in my left side of the neck. what do you think it is? no fever as of the moment but sometimes feels warm. I feel better but there are times in the days that i feel tired without doing anything. im just sitting at my office and no other activities at the moment. i used to run and do some exercises but since last wk when i have these i stop.