This also happens to me, though I am not using HRT. Mine definately has a pattern, by that I mean, certain other symptoms come along when I am feeling a bit down and different ones when I am feeling more anxious. I am still not convinced mine are solely due to hormone fluctuations and more a combination of my whole system being put to the test by the hormone changes.
I've written on here before about my storng belief that adrenals and thryoid, our insulin resistence and immune system are all 'upset' by each other at this stage of life. Some of my symptoms are clearly cycle related but others might not be and the GP's can't tell me either. Hot flushes and vaginal dryness are still the main symptoms to indicate hormones lowering to a low low point. Before that happens they dip in and out of low and high and everywhere in between - with that said - I sometimes think of this period of time as an opportunity to manage mood better. I came to this conclusion for the reason you mention, that within a day or a few days, everything is different again - it's been tough (I was extremely anxious last year and the beginning of this year, when it hadn't even occurred to me my hormones were declining duh! But now I am more accepting of the flux - I don't like it particularly, but somehow it is making me feel more confident, deep deep down, I know it is. Confidence to trust the process and to know when I need to do something and when I don't.
It really is amazing how quickly a change in mood can happen.