Sounds like water retention. It doesn't seem possible to gain weight with what you're eating, unless you have a thyroid condition or some other disorder such as Type II diabetes, which can complicate matters.
As for exercise, get in what you can. Take the stairs, park far away and walk to the store, school, etc. Or call up a friend and take a stroll around the neighborhood. You don't have to kill yourself to get a decent amount of exercise. If you can exercise, by all means, do.