It depends how severe the pain is. One can develop a painful condition (similar to
shin splints) in your forearms due to the use of a
straight bar (barbell) when training biceps. This is very rare though. For this reason many use the E-Z bar and dumbells.
You might, on the other hand, be experiencing a proper 'pump' in the forearms because of increased blood flow to the muscles when training intensely. This is actually a good sign (it shows you are training the muscles properly). When this happens you might experience difficulty to grip properly.