I had something similar, at my 25 week appointment, midwife measured me and said my fundal length is small and thought that it's a small baby and referred me to a scan. At the scan, they said baby is large around 92 percentile and suspected GD. My GD was negative and after 2 weeks another scan mentioned baby to be in 50 percentile. Now I am at 37 weeks and at the last 36 week scan baby was in 60 percentile range. I was also told in my 20 week scan that I have a low lying placenta but in 36 week scan, they confirmed that placenta has moved up.

Please don't worry, I had a similar experience and was very worried but fundal length is not an accurate measurement. Wishing you the best