Originally Posted by Flexor
This problem occurs because the elbow joint is not designed to operate under high load when the wrist is in a supinated (palms facing) position. As the elbow flexes, the wrist supinates, and as the elbow extends it naturally pronates. The natural path of elbow motion is not in a straight line and a lot of stress is put on the bones of the forearm when you use a bar.
When using a bar, pressure is put on the forearms. When you use dumbells, the pressure is put onto the biceps muscle and the biceps prevent the wrist from rotating, rather than the bones!!!
That is why I don't ever do bar curls, just db curls, whether that is standing alternating or incline supinating. The main function of the biceps is to rotate the palms towards you, and you can see this because the common tendon attaches on the inner side of your forearm. It is not in the middle, it is on the inner side so that when you contract your biceps it pulls that inner side towards you, rotating your wrist.
To avoid this problem which will only get worse, quit bar curls. Use dumbells, and try other bicep exercises, including compound ones such as underhand grip chins. You can also try the ez-bar like Gun suggests.
Whether this is splints of the forearm or not, it is hard to say. It could be the connective tissue or the bones, not the muscles. It does tend to come and go away, you are right.