fluid retention and medication, liver, kidney

I am 38 y/o female with a history of ventricular septal defect and atrial septal defect repair. I was also diagnosed with subacute bacterial endocarditis and had surgery 14 years ago to remove my tricuspid valve, where vegetation was growing. They did not replace my valve and I do have tricuspid regurg. My latest problem was chronic A fib, which has disappeared since ablation in 2/2007. My problem is I go through times where I will gain 10 to 25 pounds and lose it within 3 weeks or less. I am going through such a period now, but usually I just take a couple of extra doses of lasix and zaroxalyn (under MD advice) and it goes away. Currently Ihave gained 30 pounds quite suddenly and the extra dose of these meds is doing nothing to help. Could this be kidney or liver related? And can you develop such a tolerance to lasix or zaroxalyn that it just doesn't work anymore?

Thank you so much for the help.
Cathy