What to do when 12 year old daughter starts becoming disrespectful?

If you know, tell the truth. However, most times a 12 year old will not know why a parent thinks they are disrespectful.

You need to sit down with your mother and communicate more. Ask her how you are being disrespectful. Disrespect towards a parent is swearing at them; not following house rules such as keeping your room tidy; doing your homework; staying out late; hanging around with a bad crowd; doing drugs (not accusing you) and simply not listening to your parents. You must realize that you do not own the house your parents are in; they pay for your food; love you more than you think and if you do not respect them then don't expect any respect back from them. Respect is earned and you are old enough to start learning what it is all about.