Lots to lose - Looking for Advice

First, weight loss is not linear. In other words, your weight fluctuates, constantly. There will be weeks you lose, some you stay the same and some you gain. Your gain could easily be water retention... which could come from a whole host of things, most common being starting a new exercise, increasing exercise, excess sodium, higher than normal carbs, ovulation, period, sunburn, injury, etc.

Second, you've lost 5 lbs in 11 days. That's a great rate of loss so far.

Third, consider purchasing a food scale and weighing your food to make sure you are eating accurate portions and accurately logging.

Fourth, you should be eating at least a portion of your exercise calories back. You could probably stand to increase your calorie goal too with your current stats. However, I do understand your GP has recommended your calories, so I will just say that you need to fuel your workouts so that you can continue doing them and not deprive your body of the calories it needs to sustain your life functions on top of trying to do extra work.

I have no advice about the saggy skin though, because I have saggy skin on my stomach from my weight loss. I can only hope some of it will tighten over time, but I' not holding my breath.

So some things:

1. You're off to a good start. You may have gained in week 2, but you've lost overall. Weight loss is like that. You won't lose every week, and that's perfectly fine. Look into something like trendweight, Happy Scale (iOS), or Libra (Android), especially if you weight often, like everyday. It shows how your weight fluctuates but still can have a downward trend.
2. Take measurements. Also take some before pics. You'll wish you had once you lose some. You won't feel like you lost any, but pics and measurements will tell you a different story sometimes
3. If you're just starting exercise, expect to see some weight gain as your muscles hold onto water to repair themselves.
4. Consider planning your days ahead of time. Maybe the night before each day, log what you know you're going to eat the next day. Meal prep on the weekends makes this very easy to do. Logging beforehand helps me to know how foods are effecting my day before I even want to go snacking on my way them.
5. Log everything, never think "oh well this probably isn't that much". No no no. Log it. Also get a food scale, and weigh everything. It'll be tedious at first, but once you realize how great it is, it'll become your best friend. A lot of people eventually come on the forums and say "I'm eating within my deficit and not losing. You ask if they weigh their food. Nope, they say. Save yourself the trouble
6. Don't think of this as a "diet". That implies something you're going to do temporarily. It also makes it sound like you have to give up everything you love to eat when you don't. Don't give up on your favorites, it often results in binging on whatever you felt you had to give up. Just portion it correctly and build it into your calorie goals!

Best of luck!