Jannis,
Do you have skirts and dresses of your own stashed away somewhere and does your SO know if you do?
If you do but she doesn't know about them that might signal to her that you've been taking your dressing further and that could require further explanation.
If she agrees to you expanding your dressing getting her to help chose new items might make her more comfortable with the situation.
As often said in these situations, you know your SO best and it's ultimately down to your judgement of the situation.
