Hi Nik,
If you were taking continuous progesterone or progestogen for any length of time, more than a few weeks and then missed a day, that would potentially be enough to bring on a period. It would in me.
This is what happens when the GP would offer you progesterone to delay your period, hrt is at a lower dose that the one designed to override a young endocrine system, but many women react quickly to small changes, so the dose is already something very individual.
Whenever I've had a long delay to my period, including over 12 months delay a couple of years back, the next one is heavy and often lasts longer too.