I lost quite a bit this time and I don’t have diabetes or anything. My baby was born on 50th centile
I lost almost 10kg during the first half of my pregnancy as I was vomiting loads until ablut 25 weeks, my little girl was 8lbs 3oz when born so never effected her growth
I lost 2 stone + with my previous pregnancy due to HG, and she was perfectly healthy and fine. You've said you've cut out sugar and less carbs so it's honestly probably just your change in diet. If any concerns always speak to your midwife.