Irregular periods. Very common, usually is nothing seriously worrisome, and there is excellent help to make periods more regular and help conceiving. Irregular does not mean 'anything goes' though: if you have fewer than 6 periods a year, or if you bleed more frequently than once a month you should see a doctor promptly.
Inconsistent ovaries. Irregular periods are caused when your body's signals for ovulation and menstruation get confused by hormonal imbalances that can be related to your thyroid (TSH), pituitary (LH/FSH/prolactin), ovaries (estrogen/progesterone). Many external factors can affect the hormones: malnutrition, stress, obesity, for example. Hormonal contraception can help to regulate your periods, which is healthier.