Fertile myrtle is a slang term that is generally used to refer to a woman who appears to be very fertile based either on how quickly she is able to pregnant or the number of times she has been pregnant.
FertilitySmarts explains Fertile Myrtle
This old-fashioned term has the ability to elicit strong opinions as far as usage goes.
Fertility Can Be Personal
Fertility can be a sensitive topic. Even if it is an accurate descriptor, reproduction is still generally considered to be a personal topic that can make people feel awkward, particularly when raised by an acquaintance or stranger in a public setting.
Along the same lines, referring to yourself as a fertile myrtle can be seen as insensitive to others who may have not had a similar experience. Listening to another person speak of their robust fertility can be hard to take when you are experiencing the opposite.
And yes, to others it is simply another saying just like the bee's knees or lucky duck; a rhyming phrase that is playful rather than hurtful.