Why do I feel like this? I need help?

My sister has bipolar and im afraid i have it to, although i dont know the symptons. can someone please tell me if i have a problem, because im to afraid to ask anyone else for help. well lately ive been having mood swings, like at school i get very irritated easily and frustrated and would rather be alone then hang out with friends. i get absent minded easily and cant sit still without fidgeting a bit. and sometimes i will be quite happy and nice then other times i will be mean and short tempered. and like i said, i sometimes when im home and someone askes me to come over, i will make an excuse not to see them. and i dont know why. please help me.