I would not worry about what your mother thinks. Things have changed so much since you were young and your mother was raising you. My grandmother (my mom's mom) always gives my mom a hard time about how she is raising us. My grandmother even called me a SLUT one time!
But, we realize that she is old fashioned so we just ignore her.
Don't worry about it! Just tell your mom that you respect her opinion, but things are different now. Don't make a big deal about it and your mom will soon drop it.