Hi and thanks for your letter.
It sounds as though you haven't long been having monthly periods and I would think that your body is still making changes. It does take a while for your periods to get into a regular pattern. But as I am not a doctor I would have a chat with your mum about your worries and see what she thinks.
Have a look also at www.childrenfirst.nhs.uk this web site gives lots of information about health issues.
Hope this helps
Sam