What Causes Watery Vaginal Discharge?

Hi, Near enough every month a develop what I think is thrush as I get watery discharge, redness, soreness an a itchy vagina. It normally flares the week before I am due on my period. I had a smear test and chlamydia test which both came back normal. I use the thrush tablet and cream every time it occurs which helps and it normally goes after a week. IS there any further tests I may need or is this normal because it becomes quite distressing everytime it occurs and gets me down.