That is always what was odd about me. Regular as clockwork... until my first Check appointment. That was the first month I did not have any sign of ovulation, no positive OPK, and I think I would have had late AF if they had not started me on stuff right then. Talk about getting in under the wire! Squeak!
Weird, right? And once they got my FSH to normal (which only took 2 cycles) it stayed there on it's own for 6 months... was just starting to climb the cycle I had success which maybe one part of why, for the first time including my IVF cycle, I had three follicles going. Whatever, it worked!
(The cycle before it was up a bit and I had two but no success.) As you know, if your FSH is low and suddenly jumps it acts just like a stimulating drug.
Good luck to you and hugs,