Flu and swollen, painful lymph node on right side of neck. it hurts to swallow. how long will lymph node be swollen and causing so much pain?

Maybe infection: Can be up to a week for swelling to go down. Sounds like an infection and you may need antibiotic to help the symptoms. Rec to go see your primary doc about it.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells and digested food. Lymph nodes "taste" the lymph for anything bad like bacteria/viruses/cancers, etc. When it detects a problem, it alerts the immune system and your body reacts. They enlarge in response to the inflammation they create when they react. They shrink when all is well.