This has been covered several times before. The last time, which couldn't have been two months ago, I posted diagrams to get the point across. Some of you need to search for info before you type a question. Chances are, your question has been asked before.
Bottom line, the bones in your arm bend a little from force applied. the pain you feel would be them returning to their normal form. Weight lifting increases bone density. Your nutrition is also a factor. That means keep lifting, keep eating and eventually that pain may decrease. Another factor may be the type of curls you are doing. Using the EZ curl bar causes the two bones in your forearms to cross over one another. That can mean a better application of stress between them possibly reducing the discomfort.