Hi Bella,
I second Kahlan's comments, I have just had a BFN and AF from a stim cycle, I am day 21 today and have not yet O'd (I had a scan today showing the lead follie at 16mm so still about 4 or 5 days from Oing). I expect my cycle to be approx 39 days long, after my first two stim cycles my following natural cycles were 42 days and 44 days respectively. This happens with down reg cycles as they switch of the pituatry gland which makes your following cycle sometimes out of whack (but not all the time as my cycle in Nov following a stim cycle was only 33 days, so I guess you can get both).

Hope this makes sense. GL for your next cycle. xx