It's okay to take a pregnancy test, but it will probably come back negative even if you are pregnant. There are the "very early" pregnancy tests which might come back positive if you're pregnant, or might not.
The way pregnancy tests work is by testing the amount of hCG (the pregnancy hormone) in your urine. Generally, your body starts producing hCG when the embryo implants, and it doubles every couple of days, so by the first day of your missed period, there is enough hCG in your urine for the average OTC pregnancy test to register that you are pregnant. The "early" pregnancy tests are more sensitive, so they can show a positive with a lower amount of hCG; and a blood test done at the doctor's office is even more sensitive, so they can show a positive pregnancy even earlier.
However, different people are different, and there are even differences between pregnancies -- the first time, it took a week and four pregnancy tests before I got a real "positive" from the test; but the second time, I got a strong positive the first test, the first day or two of my missed period.
Since a pregnancy test measures the amount of hCG in your urine, the more dilute your urine, the less of a concentration of hCG there will be. That's why they generally tell you to take the test first thing in the morning -- you've likely not drunk and peed all night long, so your urine will be the most concentrated then. If you drink a lot of water, you may dilute your urine to the point that even though you're pregnant, the test registers a negative.
One caution I would give, though, is that many pregnancies miscarry very early in pregnancy, and may just register to the woman as her regular period, or perhaps a day or two late. If you get a positive pregnancy test, and then start your period, you will know that you were actually pregnant and that this is actually a miscarriage. Personal experience, here. It may be easier for you to just assume that you were not pregnant and that this is a normal period. I know the suspense is killing you! :-) But I would advise waiting until you actually miss your period before taking the test. But do what you want to do.
