I woke up with a swollen lymph node on the back left side of my neck. quite sore and slightly painful?

Our filters: Lymph nodes are our filters to take organisms (bacteria, viruses, etc) out of our bloodstream. Most common reason for a swollen lymph node is infection. Especially if it is sore and swollen, it is more likely to be secondary to infection or inflammation (it can also swell in association with allergies).

Maybe lymphadenitis: You should get seen and you may need an antibiotic for 'lymphadenitis' (a common condition of infected lymph node).

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.