Sensitivity of at-home pregnancy tests? (a very detailed question)?

[Rebecca]

I've read on the internet that most at-home pregnancy tests are very accurate if taken on the first day of a missed period and anytime after. I'm three days late now and have taken two at home tests over these past three days, with both of them coming up negative. Is it still too early? A co-worker told me they don't work until you are at least five weeks pregnant. The strange part is I didn't have sex this past month during my ovulation period. My boyfriend & I are long-distance, and we only had sex while he visited for a few days, which was at the beginning of the month right after my last period ended, so I should already be about 4ish weeks if I am. Even if I'm not pregnant, it's a bad sign that my period didn't come this month (that's never happened to me before), so I'm going to the doctor either way. How long should I wait for Aunt Flo to show up before I go see a doctor? How long should I wait to take another test?