Celia, I was going to reply to this yesterday but didn't have time, and now I see that many people have posted helpful replies and there's not much more I can add! But hang in there. It's true that metabolisms change over time, and calorie expenditure fluctuates according to lifestyle changes, and these are things that are only partially under our control. But no, just because you've gained a few pounds doesn't mean you're just going to keep gaining and gaining and there's nothing you can do about it! If you were already running for an hour five days per week, and you had a personal trainer helping you weight train, and you ate only fish and veggies, and you STILL couldn't lose the weight, then I'd say there was some metabolic factor that is out of your control! But that's not the case. You said yourself that you haven't been exercising.
The trick is, of course, to find a way to develop a fitness routine and eat healthier when you've got a young one to look after. I know how hard that can be. My kids are older now, but when my second child was a young and I was getting no sleep and didn't have the energy to exercise or remember to eat properly, I gained a bit too. Once life gets under control and you get a routine going, the weight will most likely just come back off. But it's got to be a routine you're happy to stick to, as I'm sure you know, and you can't diet. You have to eat healthy all the time, and work some treats in there so you don't feel deprived.
Big hugs! It'll all work out.