[Tiva]

Maybe you should have told your Mom before you dropped the class, they are paying for college and they feel like they have a right to know what is going on. If you had told her that you were struggling and that the teacher had told you to drop she may have been more reasonable about it because she knew it was going to happen before hand and that it wouldn't effect your GPA or your financial aid.
Your mom is treating you like an adult because you decided to act like one, now all you can do is suck up and try to explain so that she may change her mind. Otherwise you have to live with it.