Various things. Lymph nodes enlargement can be caused by various reasons- among them would include- infection ( especially in the arm pit- infection of the root of the hair is common), inflammation, wound/trauma/scratches etc, malignancy - for instance- spread from adjacent organ like breast, or lymphoma etc. Infection would be the most common, see your doc and have yourself checked. Good luck.
Problem in area. 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.