Scared for HIV exposure. Red skin under the head of my penis. No other symp. Possible exposure was 5 wks ago. Rash wouldn't just show up now, would it?

This is not HIV. HIV does not announce itself as a rash on your penis in any case. Chances are that this is a minor hygiene problem or perhaps mild allergy to clothing or detergent. If it stays around for 2 weeks or more, get it seen, or if it's painful, get evaluated now for possible herpes.
Get tested. It is not feasible to know from what you described if you are infected with HIV. Get tested. Ask your doctor for a fourth generation test. If it is negative, repeat the test in three months. See this site for info on safe sex.