AF can be a bit "all over the place" for a few cycles when it first comes back - esp if you're still BF - for some, it comes back to a nice regular 28 day cycles, for others it is much less cooperative.

for me, i went from no AF for 9 months before starting AC, to being an average of around 7 weeks now. the first one wasn't too bad, the second one was mega nasty... AF came back for me at 6.5months. she definitely hasn't gone completely awol again but it's not been regular kwim?