Potty Mouth Mya

It all started when we had my sister Cassie and my mom over for dinner a couple of weeks ago. I burnt this piece of toast to the point that there was nothing left of it. Of course all the fire alarms in the house go ragging off and the kitchen is filled with smoke when Mya found it appropiate to yell, "oh SH*@!" I just about died, it was not oh shut like she used to say all the time it was very very clear. My mom and Cassie lose it in laughing fits, I am mortified. I yelled at Mya to not say that that it was a very bad word. I honestly do not say that word, I know your thinking well she had to have heard it from someone or somewhere. Really though she hasn't. I am very very protective of the TV, she doesn't even watch TV to be honest with you, uless it is qubo in the morning for 20 min. I really don't know where she got this word! She just kept yelling it over and over and over and over again. It didn't matter that I told her to stop it was a bad word, than she started to say "oh SH*@ bad word mommy K?" It did't help that my mom and Cassie both laughed hysterically every time she said it. I was ready to smack them both. Well it's doesn't end there. Mya is still saying that word. SOme people have said well she probably doesn't know what she is saying, but she says it at the right time, like when something falls out comes oh SH*# or when she trips out comes the word again. We have started flipping her mouth when she says it, but no she is still saying it! UGH help my little 2 1/2 year old sounds like a little gutter mouth and it sounds terrible! Anyone have any ideas!