What Causes Water Retention Before Periods And Painful Lump?

Hi I am 42 yr old female. I always get alot of water retention a week and a half before my period - it has been worse for the last 3 months. I have it now and my period should be due within a week. I have just found a painful lump (feels like a bruise) under my right armpit - I know it may be a lymph node but the one under my other armpit is normal. Can you advise please?