I was the same at about 33 weeks. I had ben steadily gaining and then lost about 1-2kg over 1-1.5 weeks. I went to see my midwives and they said that your weight gain can be misleading and doesn't mean that baby is not growing well. My fundal height was still progressing so they said not to worry. The fundal height measurement is so subjective to who does it though that I was still a bit concerned, but then over the next couple of weeks I started to gain again, and now am HUGE. Not sure how much I weigh now as have stopped work where my scales were- but I started at 54kg and I think I;m 65-67 ish now (38+5). I also ended up having a scan at 35 weeks because of a single umbilical artery just to make sure growth ok and he was fine- 80th centile- so that week where I lost weight must just have been me! In retrospect it might have been because of my reflux but now am on medication for that so find eating easier. Anyway, the point is, your weight is your weight combined and not just the baby so small fluctuations do not mean anything is wrong with baby. I think they are very good at taking what they need from your body so it is much more likely to be your weight loss than baby's. I would just keep an eye on it and if you continue to lose weight see your dr/midwife, so they can check your fundal height and organise an Ultrasound if any concerns, but I wouldn;t worry too much!
Hope that helps.

LS x