you didn't mentioned what is your age, whether your weight is in proportion to your height etc.
you didn't mention your last menstruation
date also, to get positive urine pregnancy test sometimes it will take 1 week after missed date.
There are lot of possibilities for irregular periods
1. polycystic ovarian disease
, in which case you will have irregular periods or amenorrhea
2. recent fluctuations in weight,
3. any worries affecting hormonal balance,
4. dietary deficiency causing anemia etc.
You better consult your local gynecologist and get your doubt clarified.