What Causes Severe Fluid Retention In Feet And Ankles?

Hello, I have gone to 4 different doctors & have had several tests taken however they have not determined the cause of the severe fluid retention in my feet & ankles & now up in my legs as well. Also in my Stomach. I have gained 42lbs in approx 8 months & I barely eat. I have severe shortness of breathe...& the symptoms seem to be getting worse. I can not stand but for a minute or two but when I do my feet ankles turn super red & there is a burning sensation as if they are on pure fire. Please help ....