hi there
this happened with my DD.
i never had a fundus over 33 weeks when measured (not that is is acurate at late stages as baby is big and can be laying in lots of positions which alters the fundal height).

Anyhow i had the same scans for fluid etc. And it turned out that bub was just little.
SHe was born 39weeks 2 days (i was still measuring 33) and she was 5lb 13oz. (2.6kg).

Completely healthy. She was just little and still is

Try not to worry. Some babies are jsut good at fitting into tight spaces!