The weight gain could just mean that your metabolism is getting a little slower. Personally, I've always struggled with weight. But my sister, who was always a skinny minny says that she is now 10 lbs heavier for no reason at all. (She just turned 30.)
Anyway, I find that in order to lose the same amount of weight that I did 10 years ago, I have to work a LOT harder. Blech
I lost about 15 lbs since the summer and then stalled. So I hired a personal trainer at my gym ($$$$, I know), and boy, it has made a HUGE difference in 2 months. I've only lost another 5 lbs or so since I started working with him, but I have lost about 3% body fat, atleast one dress size, and am starting to get some muscle tone back. Can't wait to see what the next 2 months will bring.
Anyhoo - just my long-winded way of saying - add in resistance training. In addition to cardio and diet - that is the key, especially for us mamas in our 30s.