I'm on it and am told it can take up to 18 months to come out of your system. However, you have only had the one so maybe it won't take that long.
That said, it doesn't leave you infertile so not being able to get pregnant EVER shouldn't be an issue due to the jab. However, not being able to get pregnant IMMEDIATELY might be.
Given that your periods are regular, I would guess things are either getting back to or are normal as the whole process of bleeding, as I understand it, is entirely based on the womb buiilding up puffier walls to house an egg to be fertilised and when that doesn't happen, the bleeding is the body getting rid of those walls (and egg) and thus, it might be reasonable to suggest that eggs are being dispatched from your overies.
However, with all the above said, I'm not a doctor and I may be missing something obvious here. Why not pop along to the family planning clinic or GP to get a professional opinion.