Hi EllaM
How long have you been on HRT and what were your periods doing before you started HRT ie do you know how far along the menopause journey you are?
Tinkerbell is right - it is Tachyphylaxis that can occur if oestrogen levels consistently reach higher levels than they normally do but this tends to occur more with implants according to prof Studd:
http://www.studd.co.uk/implants.php - and he discusses it in relation to reproductive depression
If you are fairly advanced along the menopause journey then the higher doses should definitely even things out most of the time - but as CLKD says when your own cycle kicks in oestrogen levels can go temporarily higher (eg at ovulation).
Normally if you want to actually shut down your cycle you need even higher doses of oestrogen I gather.... What progesterone are you taking and how are your bleeds?
Also if you are dry down below then you should consider vaginal oestrogen in addition to the patches - many women need both so some Vagifem may well help.
I do hope you get this sorted and feel better soon

Hurdity x