Hi Ashley,
If your first period was in March, the bleeding you're experiencing now could just be mid-cycle spotting, associated with your cycles trying to get back to normal and the fact that some hormone might be still kicking around your body. Brown blood is old blood - implantation bleeding (which is actually pretty rare!) is normally a tiny amount of pink or red (fresh blood).
What I've said doesn't mean it's definitely not implantation - you could've got lucky first time! I think the thing is to not expect your body to go back to exactly how it was straight away if you've been on the depo; cycles can change lengths and it might just take a few months for it to get back to normal, once your first proper bleed has passed.
Do you track your ovulation? If you visit FertilityFriend.com, it has a really good guide as to how to do it. This has been the only thing I've been able to do to track what happening with my body (I'm basically screwed up!), otherwise I wouldn't have had a clue what was going on. Once you pinpoint ovulation and you bleed after, it's a clue of implantation or af, whereas if you haven't ovulated and you start getting the bleed you're having, you know something's not quite right (which has made sure I've got help super quick - I used to bleed like you every 12 days, gaaaaaaaaah).
Hope you manage to get sorted quickly and this bleed is a good sign
x