10 wks, u/s only showed 5 wks. miscarriage or misdate?

ljs4117

My LMP was 03/07/12, first BFP 04/26 (had a negative hot a week(ish) before).  My periods are very very irregular (ranging anywhere from 5 wks to 7 or even 8 wks apart).

I had a dr. appt 05/02 and we scheduled my first u/s for 05/16 (just two days ago).  The tech found only the gestational sac and asked if I was sure about my LMP date.  I said I was.  She said that either my dates were off or I may have lost the baby.  The physician ordered blood work to see about my hcg levels 05/16 (Wed) and 05/18 (today):

05/16 --  ~6300

05/18 --  ~10600

They still did not sound super optimistic, saying that they would have liked to have seen my levels at between 13000 and 15000.  I go in Wed for another ultrasound.

Though I am relieved to see that my numbers have gone up substantially, I'm still worried about the outcome.  It seems like I would have had to have ovulated awfully late in my cycle (though I wouldn't doubt it!!)... but yet I can see where it could be the case judging by my negative and positive hpt's.

Has anyone else gone through this??  What happened??