Bear in mind that there’s generally a lot of water loss at the beginning of low-carb diets, so you may not have lost any fat at all. Gaining mass if you’re naturally skinny can be tricky, but following Able’s advice should prevent further loss or reverse weight loss due to calorie deficit. Your gut flora can have a lot to say about your weight and metabolism, so you should see some improvement as you overcome the candida overgrowth.
You might want to read this article about potentially reversing lactose intolerance. Kefir contains more bacteria than yoghurt and a lot less protein (casein could be your issue if you haven’t been tested), so it’s a better food to try this approach. It’ll also help a lot with your candida issues. Keep in mind what he says about starting small and not giving up straight away. Any dietary change requires adjustment in the body and the gut ecology. True allergies are quite rare, and you should aim to diversify your diet as much as possible. Don’t push your guts if it clearly isn’t working, but give them a chance before giving in.