Why is my period late?

Had sex on the 25th of July, used a condom as I am not on birth control. Got my next period on the 25th of August, on the 3rd and 4th day it stopped for 3 hours each day...
Now am 10 days late!!!
Have taken a test on the 3 day and today on the 10th day and it still hasn't come, both tests where negative.
Over the last 2 weeks I also have had a lot of white cm

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