Bleeding on continuous HRT is common and happens from time to time for various reasons. I became a bit slap happy with the Oestrogel and it happened to me but I have regular TVS scans and a thin womb lining so I didn't worry about it.
The reason for the bleed can be not enough progesterone, overdoing the oestrogen like me and the progesterone doing its job and shedding the excess lining or it can mean you are getting a build up of womb lining. If you are taking progesterone every day, it's probably not excessive build up and more likely to be adjustment.
Do you have regular scans? Are you post menopause?