Parents tend to buy their kids the clothes they want them to wear from a young age. If you wore dresses and skirts before you could buy your own clothes or even if you opted to wear them for whatever reason, it still doesn't determine your gender. I wore a lot of frilly dresses as a little kid because I had no concept of fashion and really had no say in what my parents picked out for me. I don't remember caring much beyond hating having my legs exposed. Felt weird.
As I got older, I voiced my desire for guy's clothing and got to wear it, but I was still expected to wear skirts and dresses for formal things, and since I was still a kid...I didn't really have a choice there, because more often than not, the kid has to go along with their parents' desires. I've even had a period in my life where I've voluntarily worn women's clothing(haha trying to repress things is not fun, man), but that sure as hell didn't make me a girl.
If your mom's in denial, that's her issue to deal with. Roll your eyes at it and go about your business, if you can. She can't tell you if you're a boy or a girl because she's not you.