I have a hard lump on inner forearm on right arm. Been there for several years. At first didnt bother me but now since I use that arm more it get sore?

Unable.. Without looking and examining it directly we are unable to tell what it is. However, the fact that has been there for many years is reassuring somehow but the fact that it is now hurting raises a concern. See your doctor or a Dermatologist( skin doctor) to have an exam and a final diagnosis.