nsrchofbaby is right, until they see the baby on an ultrasound they will figure off your lmp date. It will change later.
I ovulated a week late, but didn't know it. They did an us at week 4, 5, and 6. We didn't see anything on the ultrasound until week 5 and it was just a sac. It didn't show a fetal pole until week 6 and that's when we found out I ovulated a week late.
Hang in there and don't get too freaked out. Keep in mind they don't usualy see anything until your hcg is over 2000. Good luck and congrats!
Your dr will normally calculate gestation age based on your Last Menstrual Period, regardless of how irregular your cycles are, until a more accurate calculation can be made based on an ultrasound. Your hcg levels will not determine how far along you are and are generally used as an outline or an average of women's levels during certain phases in pregnancy. I think you'll be fine, just continue taking whatever meds your dr prescribed. And TRY to relax and enjoy your pregnancy. Good luck, hope you have a wonderful 9 months!