period 2 weeks early - brown blood

So I’m 15, a virgin, and I got my period when I was 11. I’ve always had a very regular cycle - pretty much exactly every 28 days. But it’s just two weeks after my last period and I have dark brown blood coming out. This has never happened before - does anybody know if this is just a normal thing and my period just randomly came super early or if it’s something else? Please help :(