Can allergies be the cause of single swollen lymph node in neck? i've had it for several months and can't seem to figure out the cause

Yes: Unless this one gets bigger or others develop, don't worry.

Depends: On whether there are symptoms with it, particularly recurrent sore throats with fever. No symptoms? It needs diagnosis.

Monitoring Centers: Lymphatic system absorbs/carries things too big to go into the arteries/veins like bacteria eaten by white cells/digested food. Lymph nodes "taste" the lymph everywhere for bad stuff 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.