Why am I suddenly gaining weight?

For the past few months I've been steadily losing weight by walking 5 nights a week, usually for an hour or an hour and a half. And of course I've been careful with what I've been eating. I had lost about 30 pounds up to this week. Then, for no reason I started putting weight back on - like a pound or two every day. Now I've only gained back 4 total, but that's a lot in one week, and especially when you're exercising just as much (actually a little more) and still watching your diet. I have eaten stuffed Vietnamese Catfish the past couple of days, but that's the only addition to my diet, lol. So what the heck is going on?