Mine has come back at different times with each child, fwiw --
3 mos. with DC #1,
6 mos. with DC #2,
12 mos. with DC #3,
13 mos. with DC #4.
Is your baby sleeping for a longer stretch at night (like 4 hrs. or more), perhaps? Frequency of night nursing is one of the things that keeps AF at bay, along with nursing on demand, of course. And I agree with previous posters who suggested that it could also be a return of lochia, if you've been quite active of late or have stepped up your postpartum activities recently.
It is a bummer, though, isn't it? I had always thought absence of AF was one of the big advantages of bfing (well, okay, only ONE of them

), and was thoroughly disappointed to see her come round my door so soon. You may find, though, that AF isn't regular for many months to come, even if she returned early -- that's pretty common, too...
Kind regards,
Guin