i was on the depo shot for about a year. My periods were irregular when I was on it. For the first five months of being off depo I had very heavy periods. Then one month I didnt get my period and havent had on for five months. I still have cramping though

I have been told by some relatives that after taking the shot or any kind of bc for so long. That your body just needs time to I guess you can say time to wake up. I have also heard that bc meanly the depo shot can cause you to go into early menopause. I read on the internet (forgot which website I read it on) that there is some kind of medince (sorry if I spelled that wrong.) that your doctor can give to correct that. But not really sure if there is such a thing. I hope this helps you any.

From what i've read it can take you anywhere from 2 months to 12 months to get back into your regular cycle after you've come off birth control, particularly if you've been on it for a few years.
I think after 3 tests & 60 days without a period, it's pretty safe to presume you're not pregnant & that you're body is just trying to re-adjust to producing 'natural' hormones again (remember that when you're on the pill you're putting something 'foreign' so to speak into your body & therefore it's bound to have a big effect on the running of your body)
my advice would be that if you are concerned about pregnancy then get a blood test to 100% confirm either way. Other then that I think it's normal that you haven't got a regular cycle again & hopefully it should re-appear soon!