It could be many things hun.....Stress, change in eating habits, excersing, just about anything..,..

Check out the sticky called your chances of being pregnant.

You could have tested too early, but blood tests are usually accurate regardless. It is probably stress like the above post said. I would give it another week (try to relax and not think about it!) and if you still don't get it then give your doctor a call.

Hey, I agree blood tests are mostly always right. I think most docters will now say wait till two missed periods if all things come up neg, cos theres not much they can do basically!!
Goodluck thou chloe