Hallow Dear,
There is a lot of misconception.
Firstly, if you had menses from 10th to 15th, 18th is the day 8 of your cycle. The fertile window usually lies between day 10 to day 20 of the cycle (see below). So the chances of becoming pregnant are very remote.
Moreover, it is impossible for the nurse to diagnose a
pregnancy on day 10 of the cycle after menses. The egg is yet to be released.
The egg is released (ovulation) 14 days before the next expected menses. Egg lives for 24 hours and sperms are active for 72 hours. Hence a period of about 8 to 10 days around this day of ovulation is considered as Fertile window.
So on 20th your ovulation also had not taken place. How can you become pregnant? No test for pregnancy will reveal pregnancy as it is not there.
Please wait for the menses. If you have few more intercourse in the fertile window, you will have chances to conceive and then 8-10 days after the missed period, your urine test for pregnancy will detect it. Of course,
Beta hCG tests can give reliable results within 2-3 days of missed period.
Best Wishes,
Dr. Nishikant Shrotri