Talk to the adult you are most comfortable talking to about this and see your gynecologist. It could be a symptom of stopping the DPS but it could be many other things as well. Good luck love.
There are many reasons this could happen and it is very important that you see an obstetrician about this. The depo provera shot can alter many things about your period and coming off of it could be the root cause of the problem or it could be a thickening in the lining of your uterus and your uterus is trying to shed it. This is called endometriosis. This problem could also be hormonal. There are many factors and as I said it's important to consult your OB so that they can order tests and determine the cause. Oh and if you know you aren't drinking enough water, drink more! You need to hydrate, especially with all this extra bleeding.
Irregular periods can be really normal at your age, because your body is still going through puberty and still trying to regulate. The depo is always known to cause irregularities with your cycles, and since you've not had the jag renewed like you were supposed to - this can mess with your cycles even more. I would suggest getting a different form of birth control, though, because it seems this one could be cause of your bleeding.