Help! 8 weeks PP and no AF...

I gave birth to my first child 8 weeks ago. I never managed to get her to latch but I pumped for 4 weeks and now she's been on formula for 4 weeks. But... I haven't had my first period? I spotted around 7 weeks PP for 2 days but now nothing. We have been having unprotected sex as we are trying for another, but the 6 pregnancy tests I've done are all negative.

Advice please? What is going on?