What Causes Black Spots On Penis And Erection Problems?

Hello Doc, i have an issue with my penis shaft which appeared few days back i can see a black spot on right side of penis shaft for first two days erection was nil and then after i get it almost 50% even thou i dont touch it or rub it but during sleep i can feel erection i feel it cold i dont understand whats wrong with it this happened after masturbate few days back please suggest me something that can help me out to get back to normal thanks