Is this mania?????????????!

It does sound like Mania, but i'm not a doctor and am in no way qualified to diagnose someone, much less someone I only know from a wall of text on a computer screen.
I myself have Bipolar Disorder (Type 1...joy to the world, huh?) and I can understand and relate to multiple symptoms that you have listed.
While self-diagnosing isn't a great thing to be doing, it isn't always inaccurate. In fact, I knew long before I saw my psychiatrist that I had Bipolar, but...that isn't the point. The point is, in order to treat this and receive a legitimate and professional diagnosis, you need to get to a doctor. If your mother doesn't approve, so what? You can work around that. Talk to your school counsellor and he or she will help you work on this.

This could also just be hormones; you're only 12 years old so you're still going through puberty which is a stressful time, and hormones are capable of a lot of behavior and emotional twisting. You would have to have a pretty severe case for it to be manifesting at such a young age. When I was younger than you even, I was showing symptoms and my behaviour was incredibly erratic, but then i'm also still a train-wreck. So it is possible, but it's rare.
Just don't jump to conclusions, take things slow, and start the necessary steps to seek help.
If you need to talk, once you've indentified and recieved a legit diagnosis, feel free to email me through my profile if you need some help getting through this. I've been on medication, off of medication, on alternative treatments, and so much else, so I may be able to help you with some coping stratagies and basic conduct-tweaking behaviours.
Good luck and I hope you feel better soon.

it's very rare for bipolar to manifest in people as young as 12. it's very common for 12 year olds to have fits of happiness and silliness.

best advice is to not think about it and get on with your life. no point in getting doctors involved unless bipolar is causing serious disruptions in your life. adolescence is a special time - don't get on mind altering drugs (and some of the drugs they'll try for bipolar are seriously mind and body altering) unless you really, really need to.

Google the DSM IV-r and look up bi-polar in it... see if u fit the requirements and if you do seek a mental health professional! Good luck