Mild pain in lower forearms during biking I feel some pressure and fatigue in my wrists and lower forearms during riding but not after it. I don't know whether it is due to weakness of my wrists or my suspicion that my 18" frame mount bike is small for my 6'1" height. But the thing is that I don't feel any fatigue after biking; only during it. What do you think the problem is?
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.