Why am I missing my period??


Hey guys my name is harriet I am 12 turning 13 in September. I have missed a period and was just wondering if any of you would have any ideas of why I missed it. I have never once had a irregular period and got my first period 10 months ago. (btw I havent had sex so I’m not pregnant) so I googled and one reason could be bcs your working too hard in the gym, I went from doing no hobbies to doing a stretching class and going to the gym both once a week. Do you reckon it’s because of that or something else? I’m sure most of u know a lot more than me x also I was meant to get it about almost 3 weeks ago or something like that.