What could cause a small fundal height size for dates? I measured 33 CM but I'm 36w2d pregnant. Could it be my nutrition? Am I doing something wrong?

Many things. The shape of the uterus and the size of the baby, in addition to other things all contribute to fundal height. If the obstetrician is not concerned or making recommendations, then the smaller height may not be significant but these concerns should always be discussed with the provider. .