I know how you feel when you have had good losses then have a week which seems much lower. Sometimes your body just has to catch up with the fast loss so far. You are doing fantastically and, as Philip says, a lb of fat is a heck of a lot (and looks disgusting!) and you have lost 3! That's a bag of flour! Try lifting one of those up, it's pretty heavy.
Well done for losing 17lb in 3 weeks.
x