This is a very common situation where your body feels you are changing your habits and decreases its metabolism as a mechanism of self protection or instinct. The less you eat the slower it gets, that's our body's trying to 'protect' yourself. It should normally pass within one week or sometimes a little more. I suggest that you eat better quality food and exercise regularly but not necessarily super intense, neither eat too little. The reason; it's easier to maintain good habits if they are moderate. It's meant to be a reeducation in regards to what you eat and how you exercise.
When you start a diet it's normally advised that you try to reeducate yourself in regards to your eating habits. For instance, you could quit sugary items, fast food etc, but include more fruits and vegetables, integral and organic food and have 6 meals a day, which are divided in 3 main ones like we normally have in addition with 3 light snacks in between all of them. If you are not hungry the instinct I was talking about earlier will not be 'activated' and you'll lose weight easily. Eat a certain amount of calories, eg 1 300 kcal a day, but more importantly - eat healthy food. I once changed butter and creams, cheese etc to olive oil, included nuts in some of my snacks. Every and each of these changes make my metabolism work better and my had my health improved.
Everything that you eat in diet should technically be the same food you'll eat in few years from now. The reason I say that is because sometimes we lose a lot of weight with shakes but at some point when you stop taking them you get your weight back. You should/must include regular and daily mild exercises, and choose the right ingredients so you'll lose fat and gain muscle weight. Be patient. You'll reach your goal in a matter of time.
The best diets are the longest ones meaning lose a great deal of weight but take your time to get there, so it's more difficult to regain weight later on. I had once lost 40 pounds in 30 days when I quit taking methadone but regain the weight pretty fast. That wasn't good because I kept on getting weight until I went to see a specialist. I believe a great diet could start with a quick visit to an endocrinologist or regular physician who will analyse a set of blood exams and compare them again in 6 months or one year from now. Aim the good health because that will make you happier and more energetic.
Just a couple of things that have worked for me. When I reached my goal I realized how much it took and sometimes we have the so called 'platos' which are periods during the diet that regardless of how well and adequately you eat you remain at the same weight.
I'm sure you'll find something that is suitable for you. And this entire process can be really fun!
Good luck and keep up with the good work on sobriety!
Take care.
Erik