Ang ultrasound sa early pregnancy (first trimester) ang most accurate for dating, so that August 29 from your first scan is likely the right one, the second ultrasound can shift a bit depending sa baby's growth rate, kaya don't worry too much sa August 19 discrepancy. Pero since you're experiencing sharp pains down there na parang nagatusok ng needle, please have your OB check this during your next visit or sooner if it gets worse, because while ligament pain is normal sa pregnancy, gusto mong siguraduhin walang iba na nangyayari. A few of my mum friends had similar sensations at normal lang naman, pero better safe than sorry talaga with any unusual pain!