Taz I was just the same. Knew my cycle down to the very hour. Always knew when I ovulated (because I felt on top of the world) and was always braced for PMS from about Day 18 (and avoided anything social/important on days 22-27). Period would start of Day 28. Regular as clockwork.
These last few months it's all gone pear shaped. I can have zero PMS, but then become very, very low and anxious on the day my period starts. I can even feel very down when I ovulate. Conversely I can feel very bright and cheerful on days 18 through to whenever my period starts (could be day 24-30, who knows???).
So I no longer know where I am, or how I will be feeling. Can't plan ahead or predict anything anymore.
I am keeping my fingers crossed that this HRT will help regulate me again, so that I can predict better. And hopefully my symptoms won't be anywhere near as severe (crossing fingers tightly).