It is normal, it is actually preferred for women who are overweight to go on some type of diet, or lose weight since it can cause pregancy problems or delivery trouble, i hear the doctor i work with tell his patients this all the time.
I haven't been on any special diet and she said my weight gain was fine last week. I am due in 6 days and it hasn't caused me any problems.
The same exact thing happened to me. I went in and I was 190, then exactly one week later I went in and was 183. Same thing with me, the doctor wasn't worried at all.
I actually was researching signs that labor is near earlier today and a few of the pages i found said weight loss is a sign :)
That would sure be nice! I am miserable!