Excited... but still scared

Hi Everyone! I was at the Dr's from 11:30am- 1:45pm!!!! The appointment(more like the waiting) took FOREVER!!! My Dr's said she was overbooked!DH was not a happy camper. Well, good news is there is a little fetus with a little heartbeat!Frustrating part is that he/she is measuring at 6 weeks 6 days. According to most due date calculators I am 7 weeks 3 days. So I am not sure what to think about that. Dr. told me not to worry AT ALL! But of course, how can I not!!!I have been going over and over in my head trying to rationalize it, and I did find one due date calculator that had me enter: the date of my last period (spotting 8/12 AF 8/13), the usual length of my cycles( WHICH IS 30-31 DAYS), and it came up with 6 weeks 6 days, BUT it puts me at having ovulated on 8/31, which is 96 hours after my positive OPK!?!?!? Isnt that odd, could these tests be lying?! It makes me wonder how accurate OPK's really are now...So the appointment left me feeling excited, yet still worried and very confused. I am trying to be excited but sheesh it is so hard this time not to think something is wrong! By this time last in my first pregnancy I had already started a journal and bought a belly band ect...I have not done anything for this little one:( I HAVE to start a journal TODAY! I need to let this little one know that he/she needs to hang in there. if any of you have any thoughts or know facts on due date calculators ect...it would be greatly appreciated!