Concerned antidepressant is making my wife’s PPD worse

3 weeks ago my wife (29f, 5’5, 105lbs, just under 3 months postpartum) started an antidepressant. We had hoped it was going to help with her postpartum depression issues. She stopped nursing as she was having DMER, which helped a lot initially. However in the last two weeks I’ve noticed a continual decline. She’s apathetic, flat, and often sits silently looking blank. Sometimes she cries. She’s absolutely restricting her intake, I’ve noticed her weighing herself multiple times a day and she started buying low calorie versions of foods she normally wouldn’t (cashew milk instead of regular, low calories salad dressing, powdered peanut butter). I know her weight and that it has been dropping because our digital scale has a memory feature. She’s also taking diet pills. When I ask her she tells me she’s fine, just tired. She does not seem fine. She never smiles, she’s just going through the motions. She prefers to hand our son off to me for things like feeding and diapers because she says she’s too tired to do it. She’s told me she has no thoughts of harming him, she’s just disinterested because she’s too tired.

I’m worried about her. I am trying to convince her she should let her doctor know about these things, or at least be seen again to make sure it’s working right. She doesn’t agree.

At what point is this more serious where it would warrant me calling the doctor myself? I don’t want to overstep or betray her trust, but I am very worried about her decline.