Momies who got negative tests, but were actually pregnant?

So I have very regular cycles, i have been ttc for 8 months and this cycle i did hsg test to check if my tubes are open. After that test my doctor told me that this cycle i will have higher chances to get pregnant bc of the gel that gone through my tubes..anyways, I've been never ever late before, my period was due 15, today im 3 days late, and all tests are negatives..( I counted the days when we baby danced and it was my last day of ovulation, it was 15 days ago, is it too early to test? and how many times u got negatives before u got positive? how long did it take u to get positive? please help me ladies!