Many things can affect our cycles, stress, poor diet, illness.
If you were taking the combined pill, it may be that your periods are naturally irregular, but because of the hormones in the pill, you became accustomed to the withdrawal bleed. I bled without fail during my pill free weeks. I'm now using non hormonal contraception and my bleeds are regularly irregular, sometimes every 4th week, sometimes 5th week.
If you are sure you're not pregnant, you could see your local sexual health clinic or GP for advice, and track your bleeds for a few months, then go from there.