Welcome to HCM,
You have mentioned that you had sex during those days of ovulation,it is possible that you are pregnant but your UPT is negative please repeat the test and see if there is doubt consult a Gynaecologist and get your blood test
of Beta HCG
to know exactly.
Please do not be stressed as it is not going to solve your problem.
If required a Abdominal sonography
may be advised by your doctor and further management depends on result of your tests.
Some times stress, anaemia,hormone imbalances are the other common causes for delay but first rule out pregnancy