the dreaded three week stall - happens to almost all of us. Search this site for it - there are literally over 15,000 posts on it
just stick to your plan and stay off the scale if you have to. It usually lasts 1-2 weeks, and then you'll be on your way again
P.S. it's USUALLY the third week, but not always. Mine was weeks 2 & 3. My weight loss started up again during week four, and I dropped like 6-8 lbs in a couple of days.