Old blood

Brittany • I am 20 years old and in love with the most amazing man. While we are not married or TTC, I`m hoping that part of our life together will be soon. 😃
Today I started my period on time but unlike all my period it is starting really light, I'm only spotting and it's all old blood. The only time I have had old blood was when I was on Depo provera (My last Depo shot was August 2013) what could this mean?