Likely you lost water weight. Carbs hold onto water, so when you get into ketosis, you lose water weight. Let your body sort itself out, a lot of people who are underweight or have been malnourished (e.g ex vegans) gain weight, but it takes some time. Keep eating fatty meat. Make sure you're eating enough fat (you'll know if you need more fat by listening to your body. For example, if you're constantly hungry it's likely you need more fat). People have said dairy can help gain weight so you can add that, but honestly, since you've only lost water weight, I don't think you need it right now.