Why Do The Period Take Longer Time To Come And With Frequent White Discharge ?

Hi Doctor! I'm female, 15years old. My period take longer each time to come! Firstly, is was exactly 28 days later. And then the next month is 30 then 32days! This week, my period should come... but it haven't. I have white thick discharge recently too... My last period menstrual blood are brown. Why is this so?

I'm afraid that i am pregnant because i'm not already to!

Thanks!