I was on Depo for 4 years (and then another 6 month stint). Right after I got the shot I started my month and half long period, whihch was very heavy. I went to the doc concerned but he said it was normal. After it ended I did not have another period until I got off of it. Go to the doc if you are concerned, but really, it's the normal results of Depo. :) But TAKE CALCIUM. Depo depeletes your body of it, and you need it!
I know some people do not like it, and truth be told it has not been around long enough for a long term study, which is why I decided to get off of it. However, it was very effective for me and I seem to have suffered no ill effects, while others seem to have hated their whole expirence with it. After I was off of it a year I decided to go back (because I hated the patch) and while the results with my period were the same, I gained 50 lbs, which I was waraned by my doc might happen on a second go around. I got off it after 2 cycles (6 mo) and went tried the pill.
Some people say that you can't get rid of depo in your system for as many years as you were on it (so If i was on it for 4 I would have it in my system for 4 more years and would be able to get pregnant), or that it's a year per shot till you can get pregnant. Honestley it depends on your body. I got off depo and was pregnant with in 2 months. That pregnancy did result in miscarraiage (not related to the depo). But I was pregnant within another year, and that pergenancy was perfect. I would recommend Depo to those that can handle it. However if you are looking for long term birth control, I would recomend and IUD. After my baby I went with this, and it is by far the best BC I have ever been on, and I have been on them all.
Just remember, your body is yours, it is uniquie and can respond differently then other peoples. If you feel fine, your doc is not concerned, and you are generally happy then maybe this is the BC method for you! :)