Feeling a little defeated!

Since I got married 2 years ago I've been gaining weight and losing it on and off. It's always been relatively easy for me to lose weight when I've wanted to but after having my son last August it seems to be impossible! I'm not understanding why as I am doing everything right, I am walking loads, I've cut out sweets, chocolate, fizzy drinks and anything high in calories. I am making much healthier choices and am really not seeing results. I don't know how to get out of this rut I am in..