soft part of my arm sore

The soft underbelly of my arm (opposite from the elbow) is the most sore part of my arm today after my first workout in over three years. I remember having this same thing when i used to do curls. Am I doing something wrong, like pulling with the wrong part of the arm, or is it normal to experience soreness in this area?

If this is your first workout in over three years, you can expect some pretty good soreness for a couple weeks. It will get better.

Soft underbelly of the arm? If you're referring to the inside of your elbow (i.e., the front of your arm), that's where the connective tissue for your biceps passes, and curls work the biceps....

It's normal, especially after not lifting for so long, but you could still be doing something wrong, perhaps not warming up enough. Usually, I only get joint or connective tissue pain when I don't warm up quite enough.