Welcome here on Health Care Magic Dot Com.
I am very much concern of your problem.
I think I will certainly able to help you out of this condition.
In my view
As you are on depo so there is no chance of pregnancy
due to failure rate of depo.
And this can happen by any contraceptive
The little spotting can happen with any oral or injectable contraceptives.
This is not a sign of pregnancy.
With the depo we can’t expect even normal periods so missing of periods cannot be there to guide you about you be pregnant.
So I will suggest
That whenever you doubt of pregnancy just do pregnancy test of urine and get rid of the tension of doubt.