Leg Swelling in 25 Year Old Female

I am a 25 year old female, recently diagnosed with Multiple Sclerosis after experiencing an episode of optic neuritis, and I suffer from pitting edema in my legs, ankles, and tops of my feet. These episodes of edema are cyclic, and seem to consistently present just around the time I end my period, during high stress, flying for long periods of time, and when in hot/humid conditions. The pitting often can be graded as severe as 4+ pitting edema, and I have recently started resorting to loop diuretics (40mg furosemide) to relieve the tightness I feel in fear that my skin may ulcerate. I am very conscious of my sodium and water intake, but it seems to make no difference. I have had an echo done on my heart (both stress and resting), with normal EF, and nothing outstanding to note. I have also had my liver and kidney levels checked, along with a sonograph of them- levels and structure of both are normal. I have also had a doppler done on my legs to check for vascular/valvular insufficiently problems, which is also normal. I currently live in the tropics, but even when I return to my home in the mid west, the pitting edema, although less severe, still presents despite the much cooler/dryer temperature. What could be causing this??? I am seeking some sound medical advice, or insight from someone who may have experienced something similar to this. Thanks!