Why would i have Irregular / missed periods?

First: it is unknown. Even thin (in shape) women had irregular periods, so it could be with the hormone levels. Second: if you don't have money for a PAP smear, search for Planned Parenthood offices. They bring free services and low low services for women, even contraceptives. Third: if you are in doubt, get a blood test. Its more accurate, even the first weeks. It detects any quantity of the hormone called HCG in the blood, its more sensitive. Good Luck!