Sorry to hear about your condition. Femara
is a useful drug to prevent the recurrence of breast cancer
but can be associated with a few side effects. Fortunately, increased endometrial lining (hypreplasia) is not one of its side effects and if you are having this condition (specially 13mm), you should get it evaluated urgently.
This condition might require a pap or a hysteroscopy
Hoping for the best.