Prolonged period

Governor

So I just started birth control in October I got the shot. I got my period three weeks late and now it’s here and I’m on day 15.

Does anyone know what’s causing this, my usual cycle is 3-7 days

And now it doesn’t even look like blood or just looks brown Or like really old blood

Can someone help please!

Ps: recently got tested and I’m clean so it’s not an sti.