What Causes Fluid Retention In The Body?

Hi, my Cardiologist report shown i have a mildly dilated left Atrium. I am a 43 year old very fit female. I recently had a fluid build up in my bottom , I put on kg in 3 days with this fluid. I had been on a shart 2 hour flight on the Tuesday. weight gain accured over the next 3 days, Then i injured my calf in a running event and this is when i noticed both my whole legs and pelvic area where severely filled with fluid. My doctors did a lot of blood tests and a vein scan on my legs to check for blood clots and all my tests came back clear. My doctor prescribed fluid tablets, anti inflamatories and antibiotics for a sinus infection. After taking theses for 3 days my body returned to normal. it was ta this time that I realized it was my whole body that had filled with fluid, not just my legs. Have you any suggestions as what caused this fluid retention as all my blood tests where clear.