Hi, and thanks for your medical question on eHealth forum!
Many women get a clear discharge before periods. It starts close to ovulation and then stops and restarts before periods. A thin watery discharge all throughout is not a normal feature is generally due to genital herpes. At times the sores are not visible and the only thing that remains abnormal is a watery discharge. It can also be due to hormonal imbalance. It could also be chlamydial infection or even gonorrhea. Hence it is important to get a complete STD and a complete hormonal panel (estrogen, progesterone, FSH, LH, prolactin, thyroid) done.
It could also be due to infection in the uterus, cervicitis, vaginal infection. An ultrasound via vaginal route or a uteroscope inserted via vaginal route to look at the condition of the cervix, uterus etc, a PAP smear, a vaginal swab test, a culture of the vaginal discharge, a complete STD panel are probably what are required. It could also be an allergic reaction to soaps, cosmetics, douches, razor etc that you use. Hence eliminate them one by one and see if it helps.
It is difficult to comment beyond this at this stage. Since a confirmed diagnosis cannot be given on net please consult your gynecologist regarding this. Hope this helps. Take care!