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First of all the test will be positive a week after missed period. You have done the test early. You have had the intercourse during the safe period so chances of pregnancy
is very slim. Usually 8 th to 21 st day pof the cycle is considered as fertile.
There will always be some changes in the cycle, it will not always be the same. I think you are not pregnant and need not worry.
If you want confirm do a urine pregnancy test a week after period and also you can do the scan if you want.
Hope i have provided the information you needed.
Wish you good health.