What Causes Delay In Periods Despite Having Negative UPT?

Hi . I have some concerns . I havent had my period in almost about three months now . I took three pregnancy tests (but not all at the same exact time) in the process of the three months all weeks at a time and they all came back negative . Yesterday i had a couple blood streaks in my discharge but today there arent any . What exactly does this mean ? are the tests that i took wrong ? Or is this b/c of stress ? Or something else ? All three tests that i took werent expired .. so ..... what could it be ? Thanks so much