I have a swollen lymph node on my under arm, maybe the size of a small grape. i have bad allergies and started my cycle, will that cause the swelling?

Not likely related: Neither your "allergies" or you menses are likely causes of an auxiliary lymph node. Have your pcp assess as to any further evaluation indicated.

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.