Thanks for your query.Based on the facts that you have posted it appears that you have sustained injury
to your penis while having a dry rough vigorous sex with your girl friend .
Now your penis has become purple .Since you had no problem in having a sex next day it is unlikely to be be serious injury to corpora .It is most likely to be due to one of the superficial blood vessel (mostly vein) has been injured resulting into accumulation of blood beneath the skin .
Please get it confirmed by a clinical examination by a qualified General Surgeon
and by doing Ultrasound
scanning of the penis .
Please take broad spectrum antibiotics along with anti inflammatory medication like Diclofenac
twice daily .
Get the prescription of medicines from your family Physician.