You may have contracted a sexually transmitted infection when you had sex and did not wear a condom. You may have got it from your partner who may have had a sexually transmitted disease contracted from a previous partner. You should visit a doctor and have them check it out for you.
Red spots on your penis could be something as simple as a rash due to your detergent or poison ivy or something as complicated as a sexually transmitted disease.
Causes are; Candida yeast infection, herpes, contact allergic dermatitis, drug eruptions and pearly penile pa-pules and resulting from diabetes, STD, allergy, exposure to an antibiotic and puberty respectively.