Well, if your lmp was March 13 (if that was the day it started), then you are about 6 weeks pregnant right now. When was the ultrasound? If it was today, you are measuring a tad bit small, but that happens all the time, and its really early in your pregnancy. If your Dr. wasn't worried, I wouldn't be either. You could easily have ovulated late (I don't think anyone ovulates the same day every month), and it can also depend on how long implantation took. I also know some women measure off a bit because they have longer then the "standard" 28 day cycle. Because they don't know when you actually conceived (even if you were using an ovulation kit, it can't say for sure), they use the first day of your lmp to calculate. If you are off on your dates by a day or two, and depending on when you actually conceived, these measurements can be off. They are guidelines only. They will be looking for growth on your next u/s.
Take care, and good luck with your pregnancy...and of course, Congratulations.