First I'd like to reassure you that according to google over 40% of women get "unexplained" bleeding on HRT, it is the most common problem reported to doctors by women on hrt.
They say to report unexpected bleeding while on HRT and they seem to define that as any bleeding that first starts more than six months after a change in HRT or something very unusual going on for over three months at any time.
Definitely ask your GP for advice, especially if it's been months since the last change in your hrt. They'll probably want to check it, although reporting it to your GP is easier said than done, I know.
It's not unusual, so try not to worry about it beyond reaching out to your GP.
I'm 54, 3+years PM and I'm bleeding right now, bright red this morning, was brown smears two days ago, I had a TVScan last year which said my lining stays thin all the time, but I still bleed. So did my mum on HRT. Reactions to it can run in families, the consultant said she doesn't know why I bleed, my uterus is healthily post menopausal!