Premique is a mix of horse oestrogens, only one of which is estradiol - the one that we have in our bodies. Femoston is just estradiol - ie completely bio-identical with our own oestrogen. Therefore there is no way of comparing the dose really although it generally accepted that the equine oestrogen HRT is stronger than estradiol.
The progestogens are also different - in Premique it is a synthetic one called MPA for short, and in Premique it is dydrogesterone. This particular one is very similar to our own progesterone and is the most similar of all the synthetic ones, and therefore generally causes fewer side effects. I suspect that your uterus was reacting to the change in progestogen - and perhaps the equivalent dose in Femoston was lower, hence the spotting. This would be expected to settle so not sure why it didn't in your case although probably would eventually. Some women find that spotting never settles - because the particular combo in the tablet isn't right for them - that's the problem with taking combi tabs. Increasing the progestogen should stop the spotting if it doesn't settle - but there aren't many different types of progestogen available to take separately - and dydrogesterone is not one of them!
Which dose of Femoston conti are you on?
Sorry that sounds a bit long-winded!
Here are all the different types of post-meno HRT types:
http://www.menopausematters.co.uk/postmeno.phpHurdity x