Depo shot

So I just started the shot this month, and already my cycle is all off. I started on the day I should but then nothing for 3 days. Now she is back with a vengeance!! Anyone else have this happen? And dose anyone know if this will continue, if so just the first shot, or when I get the shot each time, or is my entire cycle gonna change? Also, is my fertility week the same or do you just not ovulate?