My lmp was march 24th meaning due date should be december 31 but after ultrasound it is january 13.

Can happen: As long this was a first trimester/early ultrasound, the due date given by ultrasound exam is certainly more accurate. Perhaps your prior cycles were irregular?

Within error: Two weeks on either side of estimate due date is within the error of measurement. Wish you good health and a healthy baby!

Fundal height: Don't worry, trust your OB/GYN, and make sure you correlate with your fundal height measurement.