Pain such as this is most often associated with peyrone's disease, nerve damage, trauma, or irritation, or a penile mass.
With peyronie's disease, you don't necessarily have to feel any masses or plaques that are usually associated with the disease - especially in the milder form. The fact that you have pain only during erection supports this cause. If this is truly the cause, treatment can involve injection of certain medications into the plaque, radiation therapy or surgery.
You should probably contact a doctor if pain persists more than the next couple of days.