My very worst day was Day 21. Actually, that entire week was really bad. I had another few days like that around the 50-60 day mark, and again just after 100 days. And none of the triggers I've read about here (holidays, change of season, etc) seemed to be the cause any of those times. But it does happen to a lot of us. Wish I could say if you just get through this time, and it will be all over 
.
But the good news is it does finally end. For some it's relatively quickly. For others like me, it took nearly 5 months. And the period of time between those hard points does keep growing. Glad you are with us. Know that you are in good company.
Some folks say that the "terrible threes" of quitting are a myth (3 days, 3 weeks and 3 months) but my experience says differently. You are at the three week mark, and it can jump up and bite you unawares. It is also at the end of what my old board called "WTF week", since it is so confusing (first week - Hell Week, second week - Heck week)... you think you are beyond the worst and "day one" jumps out again and surprises you.
This is all normal. Quitting is not an event, it is not "so over it now", it is a process, and mostly a learning process. With some trips and blips and "WTH?"s along the way, but as long as you stick to NOPE, it will all be getting better and better. Ever vigilant for those blips, but pretty soon the ease will be even easier 
. That's a guarantee, as long as you do not smoke!!
As elvan said, feelings are good. Feelings won't kill you. Smoking will. Learning to cope with feelings, without a smoke-screen to mask them, is part of the whole process we call quitting.
You're doing great!! 20 days on to 21... march!
