Hi XXXXXX
I am glad your test was negative at 13 weeks. With modern tests, being negative 3 months after exposure is very re-assuring, however there is a small percentage of patients that develop the "antibody" against HIV up to 6 months after, therefore the test can remain negative for up to 6 months (but this only happens in about 0.5 - 1% of the population).
Therefore I would suggest you getting re-tested, however it is unlikely that you are positive if you have not had any high risk sexual encounter since then, and you last test was negative 13 weeks after exposure.
Rash is a very common symptom, and can be secondary to multiple other diseases, including other viruses, which usually are self limited, and resolve on their own. You also have to take into account, allergies, particularly food allergies if the rash was diffuse, and medication allergic reactions.
I would suggest you visiting a primary care physician for a full physical exam and laboratory check up to be safe, and make sure all your cell counts and labs are within normal.
Hope that answered your question

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Your probabilities of your results changing are less than 0.5 - 1%. It would be extremely unusual for it to change.
Practice safe sex, and re-check your status every once in a while.
Take care.

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