Do I have to worry when I was not on my period for the first three months after a change of lifestyle?

Possibly. Not sure what your lifestyle change has been for the last three months, but only the most *dramatic* changes (extreme weight loss, becoming a hardcore runner, etc) might cause your menstrual cycle to stop. If you have been sexually active, you need to check a pregnancy test because one of the most common reasons for no menstrual cycle is pregnancy. If you haven't, time to see a doctor to evaluate.