What Causes Irregular Periods?

I haven t had my period properly, as in the 4-5 day cycle, for over two months. I have had my period regularly irregular for over 4 years, and have always had a slightly irregular cycle going from 30-60 days, however not like this. I have gotten my period for one day once, and just like droplets once when two people I ve been around a lot had their periods synced. I am not on the pill, but was supposed to start as soon as my next period showed, as advised by my doctor, but now I don t know wha to do, help?