I know it's worrying but the chances are it's all hormonal.
When you said it was 'believed' your periods had stopped, do you not know yourself how long ago the last one was? Is there a reason you can't tell?
Has your dr explained what happens with conti HRT? Estrogen makes the lining thicker but the progesterone keeps it in check so it doesn't bleed. It can take some months for this to be a perfect balance.
If you still have some of your own estrogen, that will prevent the balance in conti from working.
What type of HRT are you on?
I also have a thickened womb lining which has got thicker whilst I’ve been on continuous HRT.
Can you explain this? How do you know your womb lining was thick before you started HRT? Had you already had a scan for some reason?
How do you know it's now thicker? (presumably compared with a previous can?)
If the biopsy shows everything is normal, the next step is to change your HRT to one type where there is more progesterone to control the estrogen. The Mirena coil is brilliant and you'd just need estrogen as a patch/gel with it.