Doc measured belly, baby measures 3 weeks further than i actually am. negative for gestational diabetes. what other reasons cause baby to grow too fast?

Measurement error: Often, if there's no diabetes, it is a measurement error, or just a big baby. You didn't say how far along you were , but the closer to the end of pregnancy, the more variability of measurement due to the position of the baby. A trend is more important that a single measurement, i expect your physician asked you to come back a little sooner than usual to get another measurement.

S>D: Its not necessarily bad or abnormal. 90 % of abdominal measurements like these actually have fetuses with normal weight. Sometimes docs will order an ultrasound to get a weight estimate for reassurance.