third trimester weight loss

Katie
Throughout my pregnancy my midwife has been encouraging me to eat more, since I was either gaining nothing or very little. The last few months I've been doing better, gaining about a pound every 2 weeks. Trying for more, and eating  well and often. Figured I was on the right track. Well today I went for a check up and have lost 6.5lbs in one week!
Im 37 weeks pregnant. 
I truely wonder if this was an error on my midwife's part. 
Has anyone experienced this? Should I worry? 
I'll be having an ultrasound to make sure everything is ok, but so far my bump is measuring properly... 
Can anyone put my mind at ease?