Hello..... polycystic  ovarian  syndrome  is the most common  cause  for irregular  periods in these  days..  Periods  occurs  with the interval  of two to three  month  gap in between..Other factors  affecting cycle are sickness, physical and mental  stress, young age, malnutrition, etc...  Pain during menses could have secondary reason like adenonyosis,  Endometriosis etc..  Consult gynaecologist for evaluation.