First of all make a distinction between retaining water before your period and actually gaining fat. These are two seperate issues, and it is not helpful to see them as one.
About the fat gain, as we get older the rate of our metabolism gets lower, and we need to work just a little harder to maintain our weight. So this is what you should do.... Try to work in another work out during your week, preferabky aerobic-cardio exercise. Gradually increase intensity. If you do not work out at all, this is a good time to start.
