Honestly, this is what I love about weight watchers. I don't really feel like I'm on a diet....my whole life, the word "diet" was associated with hunger pangs, restriction, and deprivation. How great is it that you can lose weight and not feel that way?!?
A day every now and then at 20 points is okay. Having only 20 points every day is definitely doing yourself a disservice. You will lose weight now, but you will also be slowing your metabolism-and that will almost certainly cause you some problems later.
Also, it's possible to have a psychological effect as well. I do okay with deprivation for short periods, but eventually it bites me in the behind. Obviously we were eating more than 1000 calories a day to need to do weight watchers in the first place

So going to 20 points a day is probably a drastic change in your daily diet-whether it is in volume or type. You're much more likely to burn out on the diet or binge if the changes you're making aren't ones that you can live with long term.
I'd try to get my points in most of the time anyway. Getting those last points in could be really easy by making small changes-adding in snacks, or just eating full fat/calorie versions of what you're already eating.
We've been trained to think of food as the enemy, and that if we aren't hungry we aren't dieting.....and that is a hard habit to break. Think of those points as fuel for your metabolism so that your weight loss/maintenance is sustainable.