Hang in there! This is a great video describing what is going on:
The TLDR Version: In the first three weeks, you are losing more weight than is possible given your calorie deficiency, that is extra water around your body. You actually become dehydrated in those first few weeks until your body regulates itself and starts to store some more water to get you out of that dehydrated state. That usually happens around week 3, so you stall. You are STILL losing fat, you are just retaining more (needed) water than the fat you are losing that week.