Periods!

I am 16 and hit puberty pretty early- I started my period when I was at the age of 10.

My periods are mainly irregular (I don't have a pattern) but for my past few cycles I have been starting every 3-4 weeks which I assume is normal? But today I started my period at 5 weeks- I thought I had a routine but this has proved otherwise.

Is this normal to go more than 4 weeks? Or may I have any concerns? (I am sexually active)