After my daughter was born I didn't bleed at all- thats cause I was pregnant! Lol.
My doc calculated my last period as six weeks after my daughter was born (even though I never bled) which meant I ovulated when she was 8 weeks old.
When I had my scan they calculated I was 7-8 weeks but there was alot of confusion as we didn't know my lmp. The ultrasound later on worked it out though.
You can get pregnant more or less the month after the baby was born but the reason I didn't know was that it can take a few months for your period to get back into a routine.
After my son was born my periods were exactly like yours. Then I had the coil put in which messed it up even more, then when I had the coil took out as it was causing problems I fell pregnant again! Lol.