Hi H.,
This might start out as a round-about way of answering your question, but...trust me - I do get to an official answer! :)
When my husband & I wanted to start trying to have a child, we waited until we were on a trip to Montreal (where I used to live - loved it there & still miss it!) since I was hoping to have our child conceived there....silly, I know, but if I can't live there, I wanted *something* from there! lol In any case, we were staying at friends' houses for most of the time except for one night in a B&B...so, yep, I knew exactly what day the conception might have taken place.
Upon returning home, I took a pregnancy test exactly one week after the possible conception day, and it came back negative. I took another the next week (two weeks after the possible conception day) and it was positive. And sure enough, I was!
All that was to say...I don't know if this is true or not, but my theory is that it takes about two weeks for your body to have the hormones that the test picks up...if you take it sooner than that, it shows as negative even though you might be. (It seems like I remember reading that somewhere... perhaps on the box of the pregnancy test even??!)
Would you have an idea on when you could've conceived? If it hasn't been two weeks, perhaps you can wait a bit longer and try again? But then again, your period is 10 days late so maybe it's already been two weeks and then some....
I hate to say it - especially since you're hoping that you're pregnant - but maybe you missed your period for another reason (stress? weightloss??), but...I'll keep my fingers crossed that you *are* pregnant! :)
Sorry for the long-winded response & good luck! :)