Metabolism slowing down?

[LittleBee23]

My youngest daughter is 18 months. The weight fell off me after she was born and I've been able to keep it off easily until she was about a year. It's now been creeping back on since Xmas and I've gained 8lbs and I'm gutted as I'd finally gotten to a place where I was fairly happy with my body.
Now nothing I do seems to shift it. All the things I usually do that get results just are not shifting it at all. I'm panicking as I just keep gaining roughly 1lb a week at the moment.

I'm starting to worry that I had an over active thyroid and now it's under active?

Has anyone else experienced this?