I've Been Having Periods Every 15-17 Days. I'm 13 And

I've been having periods every 15-17 days. I'm 13 and am not sexually active or on birth control. I had regulat periods 2 years before this. I have a general appointment on Wednesday and I need advice on what it may be,what it may be caused by,how to bring it up,and what you think they'll do at the office. I'm scared to get my blood drawn so if that'll happen I need to start calming myself now haha