shoulder injury

I was lifting shoulders like 3 weeks ago and was at the gym bymyself and asked someone to spot me.  i was going for my max on shoulder presses with the dumbells i was doing the 70's and when i was getting them up initally i got them  to the top and the weight in my left arm was obvioulsy to heavy and my arm went backwords.  at first my arm felt like damn i might of just pulled something but did not hurt so i finished my workout moving back down to 60's and the next day it hurt whenever i tryed to put my arm over my head or put my left arm behind my back any idea what this sounds like?  i stayed off it for the past 3 weeks well kinda i lifted shoulders like twice but light so i didnt make it worse it is better then the first week but still seems to be a lingering pain and it only hurts now when i do the military press motion. 

Hi There, This sounds like a rotator cuff tear!?!??! ( i have suffered the same)

The rotator cuff is the name for the tendons that surround the shoulder joint. The rotator cuff is important in allowing the shoulder to function through a wide range of motions. In part due to the rotator cuff, the shoulder joint can move and turn through a wider range than any other joint in the body. This motion of the shoulder joint allows us to perform an amazing variety of tasks with our arms.
Unfortunately, a rotator cuff tear is not an uncommon problem, and these injuries make many routine activities difficult and painful. The rotator cuff is part of this mechanism that, when healthy functions very well, but when injured can be a difficult and frustrating problem.
You will be getting symptoms such as,

• Pain in localised area of the shoulder joint.
• Decreased strength.
• Difficulty in certain movements.

For treatment i would firstly take some anti-inflammatory medication, rest and consoder physio therapy.
If pain persists seek medical attention.

Good Luck
Muscular Supplements.