Period back then disappeared??

I gave birth in April, exclusively breast feed, and did not get my first period until 6 months post partum. When it came back it was a very normal period.

However, I'm now 9 months post partum and I haven't got a second period yet. Nothing, not even spotting or cramping.

Don't get me wrong, it's nice not having it, but we plan to start trying for #2 in the spring so it needs to come back eventually lol.

At what point should I be concerned/ talk to my doc? It's just so weird that it was here and then disappeared.

I have taken numerous pregnancy tests each month so I know that that is not why it disappeared.