The week predicting tests aren't exact they just give a ruff idea of how far along you are
They also date you when you see the Dr front the first day of your last period
I said what I did because you had a period, and periods generally mean the woman is not pregnant, and those home tests are only 45% accurate (in the worst case) at telling you how far along you are. Here is from the Clearblue website, as you can see it basically tells you that you can't rely on the results.
"The Weeks Estimator result is determined by the level of hCG in your urine. The level of hCG varies from woman to woman and therefore the Weeks Estimator may give misleading results. All results should be confirmed by your doctor.
"Your doctor determines how many weeks pregnant you are based on the first day of your last menstrual period and ultrasound results. This test provides a different estimate. Please note that ... pregnancy dating is dependent on the circumstances of the patient. Agreement of Weeks Estimator results with clinical findings ranged widely from 45-99%."
The amount of hCG in the urine can vary a lot (even in the same day), and all any pregnancy test does is measure that amount. I wouldn't count on the test for determining when you got pregnant, if it is important.
If you need to know for a significant reason such as determining which of two guys could be the dad, go right away to the ob-gyn and get an ultrasound, in other words, go this week or the next, don't wait because the earlier you are the more accurate the measurement will be. Ask specifically for the EDD (an estimate of your due date) based on the crown-to-rump measurement of the embryo.
Remember, doctors and ultrasound techs will tell you a number of weeks 'pregnant' that starts on the first day of your last period, not because they think you got pregnant on day 1 of your cycle but because that is where they start counting. This is why you need to count backwards from your EDD.
You will be able to plug the estimated due date that was given to you by your doctor from the ultrasound into a conception calculator, or just count back 266 days from it using a calendar, and it will give you an estimate of when you concieved that is much better than a pee test that merely takes your hCG number and matches it to some averages and spits out a number of weeks.