Pinpointing the exact date of conception is nearly impossible.
My last period started on 2/3, so my doc kept questioning how right I was because I "should be" 10 weeks along by now. Untrue since my periods weren't regular and the only dates I could have conceived were 2/10, 2/11 or 2/18. She kept questioning me today as well. But really, dating of conception is *not* an exact science.
I've read a plethora of information and sometimes the fetal pole can't be seen until 5 weeks and even 6. There's a lot of different factors, such as the experience of the ultrasound tech, the equipment that was used (older equipment wears out), your body type and so on.
Unfortunately, like me needing a second opinion - you'll have to wait it out. People keep telling me to 'keep my head up' and to think positive thoughts. I know how hard that is so I won't tell you the same thing.
Cross your fingers and wait it out is all I can advise out at the moment. Say some prayers if that helps. It's still way too early to tell anything, especially if you're only about 3-4 weeks along.