Is it possibe to BE pregnant and it not show up on a hpt?

i have been on clomid for 3 months..with no luck at ovulating..then this month i found out that yay!! I ovulated.... For the past 3 days i have been feeling soooooo queasy, and actually threw up last night, and today, i have not thrown up, gaged a few times..but still nauseous. I took a home pregnancy test a day before i got sick and it said neg...I am a week late on my period..however, i take provera, to trigger my periods..however....the last two months it started on it started on its own...i have no other symptoms, besides some insomnia, and mood changes....