Sounds like you are still adjusting to having had a baby and/or to having been on Depo, and it may be complicated by having had weird cycles in the first place. If you look around these boards, you'll see a VERY WIDE range of experiences in regaining fertility and periods. Taking a long time to return to normal doesn't mean anything's wrong.
If I were you, I'd just keep testing every 2 weeks and assume things are okay but pregnancy is a possibility at any time, and I'd talk to the ob/gyn at the usual checkup time or when the baby is a year old, whichever comes first.
If you do really want to consult the dr. now, often they will do phone conferences at no charge. Just call the office and tell the receptionist that you aren't sure if you need to come in, so could you ask dr. a few questions? Most likely they will have dr. call you back.
It's annoying when these things don't work the way they're "supposed to", isn't it?