Large Period Clumps?

So I'm currently 16 and when I got my period in the past it used to last around 5 days but now they last about 10 days and i have large almost black clumps coming out. I don't know if this is normal or not. Could someone help me out a bit? I don't use pads unless i'm out of tampons because they're more efficient and comfortable, but i used to use pads until last year.