I'm the same as you...my DS is exclusively bf around the clock, whenever he wants. We also don't use a paci and yet AF came back after just 4 months. With DD it didn't until she was 9 months old. The only difference between the two is that with DS I had an over supply and strong letdown problem so that as soon as he could he started using his thumb for most of his sucking needs. He sadly only uses boobie for food and is very fast about it, too (although he's getting better now). Anyway, that's the reason why my periods turned up early this time. I read somewhere (kellymom I think) that the volume of milk removed determines the milk supply, whereas the frequency and length of suckling determine the hormone levels. At the end of the day, though, everybody is different and different women are differently sensitive to the bf hormones. The important thing is that it won't affect your bf relationship and also, if you want to try for another liitle one, it's always better to know AF has returned that having to wait for her.
Julia, unschooling mum to Gemma (5), Benny (3) and Dom (1)