What to ask a doctor?

I think you need to give yourself a bit of a break. There is no way you gained 7lbs in one day, you'd have to eat like 25,000 calories to do that lol. Its probably water weight. You also need to remember after a baby your body is still full of some weird hormones that are going to cause your body to react differently than it used to. Also, is that 1200 calories net or gross? are you breast feeding? (you don't actually have to answer that, its just something that will effect loss), what kinds of food are you eating? with a closed diary its hard for anyone to give you any kind of feedback.

I would just be honest with your dr and yourself about your expectations and nutritional needs. it would probably even be helpful for them to see your food diary.

I think you should share your food diary with the doctor and discuss what you've been eating and how you've been logging it. Water retention is unlikely to make that much of a difference, muscle gain is not something you'd be experiencing this quickly and on a caloric deficit, and starvation mode is largely a myth aside from the final stages of actual life-threatening starvation. It sounds more likely that, because you're new to this, you may not be accurately logging your intake and exercise.

OK, so looking at your profile and previous posts, I see you aren't breastfeeding, and baby is right around 3 months old...

So, I stand with my comments about giving yourself time. Your body is still recovering and adjusting. Beyond that, we need to know height, weight, goal weight, etc before giving more advice

It is very individual. But i can say that I can definately gain 7lb of just water in a day. In my case that is mostky to do with food intolerances. When i eat grains , dairy, sugar I personally find I get extra thirsty and then swell up with fluid as what goes in does not come out until whatever irritated my system has worked its way out. My abdomin gets saw and swollen. I know that I have food intollerances though which for me cause inflamation and fluid retention. I am also a food addict with long standing binge eating disorder. I just kept on eating all the stuff i kind of knew was irritating my system because I was addicted.

Anyway my point is irritaions like that can cause large amounts of fluid retention in some people. I went on to a specialised diet plan for another unrelated neaurological problem a few weeks ago. It is basically a ketogenic style of eating with my personal food intolerances also excluded. My wieght has been consistently 22 - 23stone thats around 320lb last several months. As soon as i started new food i was peeing loads. I drank loads too but more was coming out than i could keep up with. I lost over 14lb in first few days and had lost over 20lb after 2 weeks. That was nearly all fluid. My abdominal swelling went and abdomin stopped being tender. My face, feet and generally all me lost the extra puffy look from the fluid.

I am not sugesting you have anything like as an extreme issue as me but food irritations can definately cause fluid retention for some people. Good news is it should come off just as fast as it went on as is not fat.