whats wrong with my left arm?

Everyone has a favoured side, which is naturally stronger than the other arm. You will find, expecially when doing single arm dumbell exercises, like dumbell curls, that you may not be able to do as many reps on that weaker arm. Don't worry, this won't make a difference to the results, because it works like this...

If your right arm can lift 10 pounds for 12 reps, before it reaches failure, but your left arm can only lift 10 pounds for 10 reps before failure... BOTH arms have worked to 100% of their capability, so development will be even on both.

*however, your favoured arm is always bound to be slightly bigger than your weaker arm, naturally*

I wouldn't worry about the soreness, it's probably just your left arm working harder to keep up with the right arm, because you were using a barbell. You haven't done any damage, but if you still feel pain after a few days, go see a doctor. Muscle soreness from training lasts about 2 days, especially for chest and triceps.

i'm guessing you're a righty.

its normal, but it's because you always use your right arm more than your left arm. so it's easier for your right arm and harder for your left arm.

are you right handed? the muscles on your dominant side are going to be stronger in your arms and legs

Probably because you just strained it too much. If it doesn't heal in time, get it checked.