They used to be very painful and all over the calendar when I was in my teens to thirties.
The flow would vary from haemorrhage for a week to heavy to light flow for nearly a fortnight.
There would be no warning that anything was coming until severe cramping pain started about an hour or two before the flow.
About 12 years ago, I suddenly developed iliac fossa pain. On my right side. It was severe and I finished work early to go to the GP thinking "appendix".
Nope. Mittelschmerz. Ovulation pain.
From then on, it didn't matter how irregular they were, if the ovulation pain came, then, regular as clockwork, one week later, I'd start bleeding.
Perimenopause has started, same thing. No pain, no period. For six months.
Then the pain started again. Period.
Very strong flow for the first two days, lighter for the next two, almost nothing for the last day. 5 days total now.
I also end up increasingly constipated leading up to the day it starts, then that starts flowing again as well.
However, I take antidepressants which also cause constipation, so I can't rely on that as an indicator.