My arm burns and tingles?

I HAVE A VERY BAD BURNING AND TINGLING SENSATION IN MY UPPER LEFT ARM.. BUT IT ONLY OCCURS IN CERTIAN WAYS I MOVE MY BACK FOR EX. IF I BEND BACK MY ARM BURNS AND TINGLES IT ALSO FEELS SORT OF NUMB. IT HAPPENS ALL OF A SUDDEN AT THE MOST RANDOM TIMES, AND WHEN IT DOES HAPPEN, THE BURNING SENSATION IS UNBEARABLE. AND WHILE I AM TYPING THIS MY LEFT HAND FEELS SORT OF WEAK AND FEELS LIKE ITS "FALLING ASLEEP".....WHAT IS THISS????