Hi there,
I too was on the depo shot for quite a while over the period of about 5 years and found that it took my body a while to adjust after starting it or discontinuing it. I believe its okay to be spotting and even for your periods to go away completely and then come back and normalize again as your body will not always react to it the same way. That said I'm assuming you started it as instructed otherwise it might be wise to do a pregnancy test just to be sure. Also I think you should know they've just finished some studies that suggest it is significantly detrimental to be on depo-provera for longer than 4 cumulative years (in total) as it decreases calcium absorption when you're on it. You mentioned you were on it for 2 years before so I think you're still much under the timeline but its just something to consider in a couple of years... Hope this helped and Good luck!