Depo shot

So I chose to have the depo shot before actually researching it fully. I had my first shot in December of 2015. It was also my last shot. So my first normal period was May 23rd, however it lasted 2 days longer than usual. But now today I am spotting fairly heavy. What is happening? Is this still the effects of the shot? Or is there something else happening? Not sure what to think....anyone else have a similar story??