I would recommend that you see your primary care doctor
. There are many possible causes for your symptoms -- some of which are concerning and some of which are minor and will resolve on your own. I would recommend seeing your doctor who can best identify the cause of this pain and then outline the most appropriate treatment.
As for what this could be -- it really depends on where in the forearm it is. If it is on the front of the arm (on the palm side) this could be consistent with carpal tunnel syndrome
. This occurs when the tendons that control the fingers become inflamed in the passageway that delivers the tendons from the forearm to the hand. This passageway is known as the carpal tunnel. This is caused by repetitive stress to the hand. There are many treatments for this.
Swelling in the hand can be caused by a decrease of blood return from the hand. One cause of this is a blood clot in the vein that returns blood. This is known as a deep vein thrombosis.
There can also be swelling from an infection in the skin or soft tissue. Known as cellulitis this often has overlying redness warmth and tenderness. This is a concerning symptoms.
Talk to your doctor. This warrants evaluation. Good luck!