Normally i'd say it would be because you were late but because you were only a day late and normally if you were late it would have stopped, i
m assuming it's not b/c you were late. It was probably just a timing coincidence. Being on depo (or almost any bc for that matter) can cause the lining of your uterus to become very thin. This is because you don't build up any tissue because your body sees no need too if you're not ovulating. Spotting is much more common on depo because it has only progesterone, no estrogen. So your body seemed to respond well to it atfirst, no problems with bleeding but as the lining of your uterus became thinner and thinner it almost became more suseptible to breaking easily and that's probably what's causing the bleeding. And because your body is getting very little estrogen (which is what heals the bleeding in the uterus) it's unable to heal. I'd say because you're doing so well before with it, to ask your doctor for a sort course of estrogen pills or bc pills with high estrogen to see if it will heal up. Hopefully it would stop then. If it doesn't and you can't stand it, i'd switch to taking only the bc pill, no depo, and then you'll most likely stop. You'll still have a mopnthly period but it would be better then a constant one.

I was on depo. Only got one shot. 2 week period, 2 week spotting, then a 5 week period was enough for me.

also-i've learned this from a bit of experience. You said you kept thinking you were getting it because you were late. Always follow what your gut tells you, if you think you're making up excuses, just switch to something better immediatly. I've done that a lot, thinking things would get better but to no avail.

Your body is probably just used to the depo now and is going to do this on a normal basis. Hardly anyone gets a good result with depo.