The most common causes of fluid retention are

(i) If period related : secretion of too much oestrogen also causing PMT symptoms (Agnus Castus Formula)
(ii) A sluggish thyroid (Kelp Formula, and possibly Licorice Formula (if the sluggish the thyroid is caused by or going on at the same time as sluggish adrenal glands). Incidentally this problem can also cause PMT which then causes fluid retention.
(iii) Candida (please view the 'Candida') category
(iv) Constipation (please view the 'Constipation' category).