Since last year when I made a decision to lose weight, I've weighed myself almost every day. Its interesting to see the ups and downs (occasionally I've had as much as a 5lb difference in one day - both ways) though obviously as a larger lady one pound is a smaller percentage of my weight than of yours.
As people have said, all kinds of things affect your weight on a day to day basis, and your weight changes through the day depending on all sorts of things food, fluid, exercise, clothes etc. Add into that the effect of breastfeeding too. So you should try and weigh yourself at the same time/situation each time (e.g. before breakfast, after first morning feed in your nightwear/naked).
The really obvious and (so far) unmentioned reason why your weight dips dramatically at the start of a diet (though fluid and other mentioned reasons affect it too) is that you have cut down the quantity of food you are eating, so there is less physical weight of food in your system.
The first time I ate out (huge meal) after I started weighing myself was a big shock. I went up pounds after having been losing it for weeks! The thing is, it was temporary, a few days after eating my new normal sized portions again, those pounds disappeared again (and I had still lost weight.. graph continued general down trend, it didn't level then begin dropping again). It was the sheer weight of it in my system, not a permanent addition. Obviously if you normally eat large portions then your system is fuller, and you will also store some of those extra calories if you are gaining weight too.
So that first weigh in after eating less includes a drop in the quantity of food you are digesting at any one time.
Obviously keep an eye on it, if you keep losing weight faster than you expected and are comfortable with then maybe you've cut out too much and need to eat somewhere in between your old and new normal to maintain/lose weight more gradually. Remember that your maintence requirements will change depending on your activity levels, breast-feeding etc. So once you are at your comfortable weight that you are maintaining, if you then stop breastfeeding you'll need to slightly decrease your food intake to stay the same.
Obviously get checked over by your GP if you are still worried and/or advice from a nutrionist about healthy eating.
Again as Oogaballo suggested, you might have been eating more unhealthily before than you thought, just because you weren't focused on it, so your healthier diet might be more of a change than you expected it to be.
This is a bit of a stealth boast though! 
I'm pregnant and my weight loss is on hold (can't wait for that first post-birth weigh in week!).
Daily weighing in isn't usually recommended as it can make some people obsess about losing weight (because they want it to go down each time). I haven't found that, I just find I am more mindful about sticking to my new healthier habits and a bit fascinated by the general downward trend despite daily ups/downs.