Hi Op, hope you are okay. Sounds like a stressful time for you.
I had sertraline when I was at my absolute lowest. I was an idiot and only took for a few months. I found they helped level me. They can make you very tired the first two weeks, and sometimes you can feel worse before you feel better, again, particularly in those first two weeks. Just make sure you have your loved ones at hand and speak honestly and openly with them as much as you can through this.
Please, please, please don't worry about weight gain! You've had a baby! I know it's so hard not to beat yourself up and find flaws, but I guarantee you're doing great. You have a little person who loves you more than anything, whatever weight you are. You are their whole world! Cut yourself some slack and focus on getting better for your LO, and for yourself.
Perhaps, if it's an option for you, go and buy yourself a few items of clothing that make you feel good. That might help your confidence perhaps?
I never had weight gain with AD, but I'm one of those annoying fucks with a stupidly high metabolism.
Only think I found with sertraline was at too high a dose my legs would sometimes go jittery, say if I was driving and I was holding the bite on the clutch or whatever, my legs would just do weird things. But halving my dose helped.
They also made me shake whilst coming off them, particularly at night. I found just taking one when it got too much was fine and before I knew it they'd stopped. They have a small half life so when coming off withdrawal symptoms can be fast to kick in.
There are so many AD out there that work in different ways. If after a few weeks it's not working enough, or if it doesn't suit you, go back to your GP and try something else.
Good luck OP. As someone who felt like I would never get out of that dark pit, I promise there is light at the end of the tunnel. Just look after yourself, and be kind. Eat the chocolate, have the nap and do what you need to do to feel better. Don't feel bad about relying on your loved ones more. Help is okay.
Sending lots of love and support x