Why does the doctor always have to ask me about my period?

I'm 13 and yesterday I went to the doctor because there was something wrong with my ear. They just had to ask me when I had my last period like always. The first time I got asked that question, it was the actual doctor ( who's a man) and this time it was the nurse. What really sucks is that my mom was in the room when they asked me and now she will know when I'm having my period every month. That's not good because now my mom is gonna be all in my business and she's gonna be asking me if I'm wearing a pad before I go to school and stuff. My mom is so snooty. When ever she knows I'm on my period she has to tell everybody too! I can't stand her! Why does the doctor always have to ask about my period even when it has nothing to do with why I'm there?