Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km (18,000 ft) of the troposphere. It is primarily composed of nitrogen (78%) and oxygen (21%) with only small concentrations of other trace gases.
Having irregular periods is very common. I see them in my practice all the time. The causes can range from something insignificant to something that requires treatment. For example, if you're a teen, your body's hormones can fluctuate and take some time until they find a balance.
Pelvic inflammatory disease (PID) and other infections can cause irregular periods, as can endometriosis. This is a condition in which tissue that lines the inside of the uterus begins to grow elsewhere inside the body. This can cause heavy bleeding, as well as pain.