There is a possibility that this could be a clotted vein due to a rupture which ovvured earier and was unnoticed. This condition is not uncommon and usually resolves itself with abstinence and use of local anticoagulant
creams but if it does not or if you have pain you might require tests like an ultrasound
study of soft tissues of your penis and maybe a colour Doppler examination of blood supply to the penis as well.
Hope I have been of assistance.