Mild pain in lower forearms during biking
Fatigue indicates you are using the muscle, so you will need to “check into” yourself to see how/why you are firing your forearm muscle, what task is it trying to accomplish.

Sometimes we just have some odd or peculiar habits we are unaware we are doing. For example, my wife used to complain about wrist and hand pain when riding her flat bar commute bike until I observed she held her wrists in heavy dorsiflexion when she gripped the bar. Retraining how she held flat bar grips (i.e., a wrist angle of zero) alleviated the symptoms.



Finally, as other answers indicted some component of bike fit could also be the culprit, but there is not enough information to comment on this possibility.