You do get false negatives if you test too early, but they are pretty good (I think) if you are 14 days + past ovulation.
Is your 16 month old nursing more than usual or anything like that? I would imagine that could throw your cycles off a bit. It is very frustrating, though, especially when you do want to get started on #2.
Chances are, your GP will put your odd cycles down to the breast-feeding...I had very weird cycles when AF finally came back (at 13 months post-partum), and that's all my GP said.