First of all let me say how jealous I am! 18 mos. and no AF!!

Mine returned at only 8 weeks p/p despite nursing on demand and no paicifier!

: ANYWAYS...I had REALLY wacky cycles with dd2 and had a couple months where I though OH NO!! I got AF back really early with dd1 but it was ALWAYS on time every 28 days, so this time it really threw me thru a loop. It took about 6 or 7 months for them to get back to normal, but one cycle was 40 days! That was the one time I was freaking out because I though how could I go from having a very regular cycle to this!

So I wouldn't test just yet.

HTH.
ETA: Does your ds still nurse often? Although I have heard of women whose children have weaned and they still don't get AF for another 2 or 3 mos.!