I am sorry about the delay.
There can be a few reasons for a delay in menstruation, these being pregnancy, stress, anxiety, diet changes, contraceptive pills, medication etc.
The first one that seems more likely in your case is pregnancy - even though the pregnancy test was negative.
I understand that a late period and recurring negative pregnancy test results can lead to a great deal of stress, worry, confusion, and conjecture on what might be happening.
Assuming that you are pregnant, the most simple explanation is that the test you have purchased from the drugstore is of a lower-sensitivity. This means that the test is capable of detecting pregnancy only when the level of hCG reaches a certain threshold, and the test is simply not sensitive enough to yield the positive result. Follow the instructions of the pregnancy test kit and give the kit the required reaction time to induce the results.
Another reason is when you have had a lot of fluids to drink, this can dilute the amount of hCG in the urine and bring about a negative test. It is always best to use the morning sample of urine and collecting the mid stream urine for the test.
Another reason is that hCG develops at different rates among women. Some women can easily discover if they are pregnant well before their missed period. Other women may need to wait a bit longer.
If you have missed a period and are experiencing pregnancy symptoms like morning sickness, sore or tender breasts etc then it is best to get a hCG test and an ultrasound from your doctor to rule out ectopic pregnancy or any other problem with the pregnancy which might be causing lowered production of hCG.
Other reasons such as stress, anxiety, previous problems with periods, diet changes have been ruled out in your case by the history that you have provided. Less important causes of missing a period are vigorous exercise, miscalculation etc.
Keeping in mind that you have had unprotected sexual intercourse and were not on any contraceptive medication during this time, this could be due to conception, and I suggest that you get a sensitive pregnancy test kit from the pharmacy and perform the test again after a couople of days.
If you still yield a negative test and continue to go without having periods then you should visit your PCP for further tests and a thorough work up.
Hope this helps.
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