Well, I don't know how old you are, but typically, girls and women can experience irregular periods even when nothing is wrong. Some women have it easy, their periods regulate and stay on the same cycle, same days, and same PMS and pain during periods every month. But some of us aren't so lucky. Last month I had two periods within a week of each other, light ones, and sometimes I have regular periods, sometimes I almost skip a month, my periods are so far apart, and sometimes my periods last for up to two weeks. Some women just don't stay regular their entire period. and that could be the case for you. Just because nothing is showing up on your tests doesn't necessarily mean that's not the issue here. All I can tell you is to track your periods each month. Write down the day that start, the day they end, how light or heavy they are and what PMS you are experiencing, along with just regular cramps, blaoting, irritability, etc. If something starts to get extremely odd and unsusual with your periods and you feel the need to panic, then go to the doctor's immediately. Otherwise, I think it's just regular old irregularity =) good luck.