As far as I know, most feminist and feminine girls/women I've known my entire life say "femininity is a concept." This is a motto I'v adopted growing up in a femal environment. The most tomboyish female in the world could also be considered the most feminine if her ideals and concepts relate to how she views herself and everyone else, male or female, as a whole. If you mother says that you aren;t feminine enough, analyse for yourself how YOU depict femininity, and analyse how your views of this is encorporated in those around you.
I'm going to go jsut a bit off topic and take a video game example. Princess Peach and Princess Daisy are neither living nor real individulas, but they were made by rela people. Both of them are princesses...a feminine title by itself. Yet they bother act in non-feminine ways when they are out of the normal scheme of "Damsels in distress." They fight, they throw fits, they play sports, and they even rescue thier boyfriends. Now this relates to my previous point of feminine aspects of a person. I KNEW people who were like this in my highschool. They were more rough and tumble than I'll ever be, and they were quite feminineif not loud and rambunctious at times.
So, as you can see, the concept is a set standards of PERSONAL idealisms for yourself, not what others think of you-though i wouldn't go so far as to say there are irrefutable aspects that others percieve about you that you SHOULD take into consideration, such as the tale-tale blatantly physical traits and actions that you would find all-to-rarely in a girl, such as the comical (yet very real) hand-down-the-pants-for-no-reason pose on the couch, or the act of trying to purposefully gross someone out. These jsut don't fit (for obvious reasons)
As for your mother...well, I'm not the sharpest knife in the drawer, but she may just have a differant idea (or generalized societal idea) of what it means. Who knows? Oh, I'm rambling on again.
I hope I was of SOME help to you, Aelita.