I just need a bit of advice

I'm 15. I only got my first period in December. They've all been really regulars once then but I've missed two in a row now. I wasn't sexually active after my last period but I've taken two tests just in case and they're negative. I'm just concerned as to what it is that's causing it. It was regular and it's kinda worrying. I just need some reassurance that it's nothing too serious or anything. Thanks guys xxx