Hi Liz,
I am on Femoston 2/10 and have had mid-cycle bleeds twice (i.e. Before or just at starting the combined tablets). Then a regular bleed at the end of the 28-day cycle, too. This worried me but after Dr. checks nothing wrong was found. I was given additional progesterone to take Day 11-25 which resolved the disbalance. I stopped the additional prog after two months. Since then, there hasn't been any problem.
I think with HRT and peri, your own cycle can go out of synch with the 28-day cycle of HRT. This results in unscheduled bleeding, as your own estrogen/progesterone dynamics can be sometime stronger than those of the exogenous hormones. Nothing to worry about! If you continue though to have frequent unscheduled bleeds, consult with your specialist how to bring your cycles on track.
All the best
Milamam