These words, that your mom said,
"You were so girly as a child"
"Being trans is very trendy right now"
Indicate that she does not appear to understand what being trans is really about.
Has she been exposed to informed advice about what being trans actually means?
I would start at that point.
If you are uncertain as to the sources of your mom's sources of information, send this link to her before you even start a conversation. And then just let her ask you questions. Maybe do this over email so that there is a bit more thinking involved and less knee jerk reaction.
http://cupola.columbia.edu/978-0-231-15712-4/29It's also important for you to remember that your family are transitioning with you, and they have probably spend a lot less time thinking about the transgender concept than you have.
Have you seen a therapist? If so, your mom should be invited to attend a session.
Take it slow. One baby step at a time.