Don't forget that when you start eating right and hydrating yourself that your body stops storing water, so I would imagine some of that weigh is water that your body figured out it didn't have to hold on to anymore since plenty of it is coming in.
So I wouldn't be worried about the big loss in the beginning, it happens to a lot of people that it falls of when you start.
Just be stoked that you've done so WELL and are developing excellent habits!
Happy birthday and congrats!