You should check with your dr. I was on Prometrium (progesterone) from 2dpo and through my 1st trimester, so that shouldn't be a problem. A neg blood test could be mistaken, IF it was done too early. Best wishes.
Glenda is right, progesterone should be fine if you are pregnant. Many of us take prometrium through first trimester or after ovulation. False positives are uncommon, but I can tell you that the blue dye tests seem to produce them more often then the pink dye. I would see your Dr. for a blood test. I wish you luck!!