Weight loss during third trimester?

I am 37 weeks pregnant, I had stayed at the same weight for 8 consecutive weeks, and then when I went to my appointment today, I had lost 6 kilos since my last appointment. This means I lost about 13 lbs in two weeks. Is this normal? I hear of people losing 1 or 2 pounds, but not 13 pounds. My doctor wasn't quite sure what to think either..