AF can do all kinds of weird and wonky things when you are BFing, and I think pretty much every mum has been in your shoes worrying about another pregnancy so soon! After Marieke was born, my first period was very heavy, whereas after Emily, it was at about 5 1/2 weeks post partum and fairly light. Since then, I've had some months relatively normal, some missed, and some really light. Get a bunch of pg tests for those stressful days, and POAS to ease your mind.
All the best!