If you had a blood test before starting the pill, I would take that as being pretty accurate. I'm not sure exactly how early it can detect pregnancy, but I know it's far earlier than a hpt, and at 10-12 days post-conception, that's pretty early to begin with! You probably wouldn't begin to feel any pregnancy symptoms (especially if you've never been pregnant before) until at least 4 weeks after conception (however, everyone is different, so it's possible to have some symptoms sooner--like frequent urination!).
If you're really concerned, I would ask the dr for another blood test to confirm that you're not, but the symptoms you're experiencing are probably from the pill. I was on that pill awhile back, and had a whole host of side affects.