I'm 25 and I started my periods on my 9th birthday. Honestly, they where very irregular for the first 2 years and I couldn't really put an exact date to when my periods where due. This is normal as your body is readjusting to the changes that naturally occur as you grow older. Your hips are getting wider, your putting on weight, your breasts are developing and obviously raging hormones too. These are all part of growing up, and periods are no different.
It will take time for them to settle in, but give it time and they will become more regular.
The only reason you need to visit a doctor is if you've missed your period completely for 6 months in a row, as this may be a sign of a hormone imbalance, or another health issue associated with you menstrual cycle.
Hope this helps
x