HELP ME PLEASE!! Something is wrong with my arm!?

On 16/04/11 , sat , i was doing some work out on my left bisect . It's been month since i last workout so i expect my muscle will ache for around 4 to 5 days . However, my left arm could not be straighten due to sort of cramp feeling on my left bisect so i though it's just normal aching and let it rest for a week. Today , i try working out my left bisect @ my place. I realised that i can't feel any power in my left bisect , when i try lifting up a 3 kg dumbell , it seems like i lifting a 12 or 15 kg dumbell . While my right bisect can lift 12 kg dumbell easily. Can anyone tell me what's wrong with my left bisect . I'm 18 this year D: