I'm 34, 3 kids aged 12, 8 & 7. Cycle has always been 28 days, now in recent months has changed to 23. Changes in flow etc. Is this normal? Causes?

Shorter cycle. Sounds like low Progesterone or luteal phase defect before your period. This causes periods to start sooner than usual. If it persists, you may need a Progesterone boost.